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Dalton
Transactions
PAPER
Cite this: DOI: 10.1039/d4dt01886a
Received 29th June 2024,
Accepted 7th August 2024
DOI: 10.1039/d4dt01886a
rsc.li/dalton
High-pressure polymorph of Co
3
P
2
O
8
: phase
transition to an olivine-related structure
Robin Turnbull,
a
Josu Sánchez Martín,
a
Akun Liang,
a,b
Daniel Díaz-Anichtchenko,
a
Catalin Popescu,
c
K. Sandeep Rao,
d
S. Nagabhusan Achary,
d
Alfonso Muñoz,
e
Vinod Panchal
f
and
Daniel Errandonea *
a
The monoclinic polymorph of Co
3
P
2
O
8
(space group P2
1
/c), isomorphic to farringtonite (Mg
3
P
2
O
8
) type
orthophosphates, was studied up to 21 GPa using synchrotron powder X-ray diffraction and density-func-
tional theory simulations to investigate the influence of pressure in the crystal structure. This study
revealed a pressure induced structural phase transition for monoclinic cobalt phosphate, Co
3
P
2
O
8
, and
the details of crystal structure of the new high-pressure polymorph were delineated. The evolution of
XRD pattern with pressure indicate that the onset of a phase transition occurs around 2.90(5) GPa, and
the low- and high- pressure phases coexist up to 10.3(1) GPa. The high-pressure phase also has a mono-
clinic lattice (space group P2
1
/c), and a discontinuous change of unit-cell volume of about 6.5% occurs at
the transition. A reorganization of atomic positions with a change in the cobalt coordination sphere
occurs in the phase transition. Notably, the high-pressure polymorph has a defect-olivine-type structure
like chopinite-type orthophosphates. Using a third-order Birch–Murnaghan equation of state, the bulk
moduli of the low pressure (LP) phase (75(2) GPa) and high-pressure (HP) phase (92(2) GPa) were deter-
mined. For the low-pressure polymorph, the principal axes of compression and their compressibility were
also determined. Density-functional theory calculations also provided the frequencies of Raman- and
infrared-active modes which can be used for mode assignment in future experiments.
I. Introduction
The A
3
B
2
O
8
-type orthovanadates and orthophosphates (A =
divalent cation) show varieties of structure types depending on
the ionic radii of the A cation as well as preparation conditions
or external thermodynamic parameters like pressure and
temperature.
1–3
Owing to the presence of isolated vanadate or
phosphate groups, they often form loosely packed and flexible
crystal structure, and hence varieties of functional properties
relevant for several technological applications like laser-host,
phosphors, scintillators, display devices, ion transportation,
and catalysis or photocatalysis, etc. can be expected or intro-
duced in such materials.
4–9
For larger divalent cations (A
2+
)
like alkaline earth or lead cations, the A
3
B
2
O
8
type vanadates
or phosphates usually form palmierite, apatite and related
structures.
2,3,10
Such compositions can have centrosymmetric
or non-centrosymmetric lattices. They are known to show high
dielectric constant with low loss character, microwave dielec-
tric properties, and ferroelectric properties. For the smaller,
A
2+
ions, they form relatively denser and distorted kagome
structures, and hence display severely altered electronic pro-
perties and optical properties depending on the nature of A
2+
ions.
7,8,11–16
Rich polymorphism under pressure with the existence of
newer phases and contrasting sequences of phase transitions
depending on the divalent element A has been documented
for several of such A
3
B
2
O
8
-type vanadates and phosphates.
This fact has been well understood in the compounds of ortho-
vanadate family.
11–18
Among these compounds, Cu
3
V
2
O
8
decomposes into CuO and V
2
O
5
at 1.35 GPa,
13
while Ca
3
V
2
O
8
,
Mn
3
V
2
O
8
and Sr
3
V
2
O
8
, undergo phase transitions at 9.8,
15
10,
16
and 13.8 GPa,
2
respectively. However, Zn
3
V
2
O
8
,Ni
3
V
2
O
8
,
Co
3
V
2
O
8
, and Ba
3
V
2
O
8
do not exhibit any phase transition up
to at least 15,
14
23,
12
20,
12
and 29 GPa,
3
respectively. In con-
trast to their vanadate counterparts, only limited studies on
a
Departamento de Física Aplicada-ICMUV, Universidad de Valencia, Dr Moliner 50,
Burjassot, 46100 Valencia, Spain. E-mail: daniel.errandonea@uv.es
b
Centre for Science at Extreme Conditions and School of Physics and Astronomy,
University of Edinburgh, EH9 3FD Edinburgh, UK
c
CELLS-ALBA Synchrotron Light Facility, Cerdanyola del Vallès, 08290 Barcelona,
Spain
d
Chemistry Division, Bhabha Atomic Research Centre (BARC), Trombay,
Mumbai 400 085, India
e
Departamento de Física, MALTA-Consolider Team, Universidad de La Laguna,
San Cristóbal de La Laguna, E-38200 Tenerife, Spain
f
Department of Physics, Royal College, Mumbai 401107, India
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the HP behaviour of the analogous phosphates are reported in
literature.
19–23
Information on the high-pressure behaviour
of such phosphates and the structure of high-pressure
phases are mainly available on the minerals, like, sarcopside,
(Fe,Mn,Mg)
3
(PO
4
)
2
, chopinite, [(Mg,Fe)
3
](PO
4
)
2
, and related
orthophosphates.
19–25
It has been suggested that these denser
polymorphs have a defective olivine-type structure and are
formed by combined effects of temperature and pressure on
farringotoinite by a meteoritic strike.
23–26
Thus, they also can
be probably synthesized by HP-HT techniques as metastable
phases.
23,26,27
Most of these studies were focused on Mg
2+
or
Fe
2+
containing orthophosphates due to their mineralogical
relevance. However, to the best of our knowledge analogous
orthophosphates with smaller transition metal ions are not
explored under pressure. It is well known that pressure not
only affects the structural properties but also modifies bond
distances and angles in the structure. Thus, it can be used to
tune the vibrational and electronic properties conveniently.
28
In some cases, application of pressure can lead to the discov-
ery of new metastable phases with interesting physical pro-
perties that can be achieved at ambient conditions, and hence
can open new opportunities for applications.
1,29
To under-
stand the effect of pressure on a transition metal ion bearing
phosphates, a systematic study on cobalt orthophosphate,
Co
3
P
2
O
8
, under high-pressure conditions up to 20 GPa at room
temperature was undertaken.
Co
3
P
2
O
8
crystallizes in a monoclinic structure described by
space group P2
1
/c(it is also described in the literature by the
non-standard space group P2
1
/n).
30
It is isostructural with
γ-Zn
3
(PO
4
)
231
and Mg
3
(PO
4
)
2
,
32
farringtonite-type orthopho-
sphates.
30
The crystal structure of ambient pressure phase of
Co
3
P
2
O
8
is depicted in Fig. 1a. In the structure, cobalt atoms
occupy two different coordination polyhedra, a slightly dis-
torted octahedron (CoO
6
) and a penta-coordinated polyhedron
(CoO
5
), in a ratio of one to two respectively. The penta-co-
ordinated CoO
5
polyhedron can be described as a distorted tri-
gonal bipyramid. The presence of such polyhedra in related
vanadates, like Zn
3
V
2
O
8
, has been related to the existence of
pressure induced phase transitions.
14
Zn
3
V
2
O
8
undergoes a
phase transition by hydrostatic pressure below 4 GPa, which
has been attributed to the displacement of an oxygen atom
towards ZnO
5
units under the influence of pressure and that
leads to a closely packed structure and transforms the ZnO
5
units to ZnO
6
octahedral units. We hypothesized that such
structural changes could be a common phenomenon in such
orthovanadates and phosphates, and that further motivate us
to explore the high-pressure behaviour and possible pressure
induced phase transitions in Co
3
P
2
O
8
, which has not yet
been studied under compression. In fact, Co
3
P
2
O
8
is also
scarcely studied at ambient conditions and more commonly
investigated in the interest of its use as a pigment in cer-
amics, (it was the favourite violet pigment of Claude Monet
who used it in the famous Water-Lilies painting
33
), as a catalytic
material,
34
as an electrode in super-capacitors when hydrated,
35
as well as due to its interesting magnetic properties.
36,37
However, neither experimental nor theoretical data on its
mechanical properties or lattice vibrations are reported in
literature.
To understand the properties and HP behaviour of
Co
3
P
2
O
8
, we have performed a combined in situ HP powered
X-ray diffraction (XRD) measurements as well as density-func-
tional theory (DFT) calculations. A structural phase transition
was discovered at 2.90(5) GPa, and the crystal structure of the
new phase has been determined. Compressibility, elastic con-
stants and bulk moduli for both ambient and high-pressure
phases were obtained experimentally and theoretically. The fre-
quencies and assignments of Raman- and infrared-active
modes for both ambient and high-pressure phases have been
calculated, and to our believes these are being reported for the
first time.
II. Materials and methods
Co
3
P
2
O
8
was prepared by a multistep reaction of Co
3
O
4
(Alfa
Aesar, 99.9%) and (NH
4
)
2
HPO
4
(Thomas Baker, 99.5%). All the
reactants were used as received. The weighed quantities of the
reactants were thoroughly mixed and heated at 300 °C on a
Fig. 1 The crystal structure of (a) the ambient-pressure phase of Co
3
P
2
O
8
, and (b) the high-pressure phase of Co
3
P
2
O
8
. The CoO
5
and CoO
6
poly-
hedra are shown in blue and the VO
4
tetrahedra are shown in grey. The red circles represent the oxygen atoms. The unit-cell is represented with
black solid lines.
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hotplate using a platinum crucible. The mixture was heated
until the complete melting and decomposition of (NH
4
)
2
HPO
4
was ensured. Once cooled, the solidified mass was then hom-
ogenized, pressed into pellet and then heated at 750 °C for
12 h. This step was repeated after homogenization of the
product. Further the sample was crushed and pressed to pellet
and heated at 900 °C for 5 h. All the heat treatments were
carried out in air using a platinum boat as sample holder. A
crystalline single phase monoclinic Co
3
P
2
O
8
was confirmed
from powder XRD measurements of the sample. The ambient
pressure unit-cell parameters observed from the powder XRD
data are: a
0
= 5.0629(5) Å, b
0
= 8.3611(8) Å, c
0
= 8.7882(8) Å,
and β
0
= 121.0(5)°, and they agree with crystal structure
reported in literature.
24,30
High-pressure angle-dispersive powder XRD experiments
were performed at ambient temperature at the Materials
Science and Powder Diffraction beamline (BL04-MSPD) of
ALBA synchrotron.
38
We employed a membrane-type diamond-
anvil cell (DAC) with diamond culets of 500 μm in diameter. A
200 μm thick Inconel gasket, pre-indented to a thickness of
50 μm, with a 250-μm diameter hole drilled in the center was
used as the pressure chamber. A mixture of methanol–
ethanol–water (16 : 3 : 1) was used as the pressure-transmitting
medium (PTM). This PTM remains quasi-hydrostatic up to 10
GPa,
39
but it has been successfully used to study high pressure
behavior of oxides up to significantly higher pressure than that
of the present study.
40
A Cu grain was loaded next to the
sample in DAC to use as pressure marker. Pressure at the
sample was determined from the position of the Cu 111 reflec-
tion in the XRD pattern and the equation of state (EOS) for Cu
reported by Dewaele et al.
41
During the process of sample
loading, great attention was given to ensure that only a minor
portion of the pressure chamber was occupied by the loaded
sample and Cu. This was done to minimize the likelihood
bridging between sample and diamond anvils. For HP XRD, a
monochromatic X-ray beam of wavelength 0.4642 Å focused
down to a spot size of 20 × 20 μm (full-width-at-half-
maximum) and collimated with a pinhole. XRD images were
collected on a two-dimensional Rayonix SX165 CCD detector.
The two-dimensional diffraction images were integrated using
DIOPTAS.
42
Rietveld refinements were performed using
FullProf software package.
43
Total-energy ab initio simulations were performed within
the framework of density-functional theory, DFT,
44
with the
Vienna ab initio Simulation Package, VASP.
45,46
In this study we
utilized projector augmented-wave (PAW) pseudopotentials
47,48
and extended the plane-wave kinetic cut-offup to 540 eV to
guarantee highly converged outcomes. The integrations over
the Brillouin zone, BZ, were carried out with k-special points
samplings (6 ×4×4and4×6×6grids for the low- and high-
pressure phases, respectively). The exchange–correlation
energy was described by means of the generalized-gradient
approximation, GGA, with the Armiento and Mattsson, AM05,
prescription.
49
To treat the strongly correlated states appropri-
ately, the DFT+U method of Dudarev et al.
50
was employed.
This method utilizes a single parameter, U
eff
=U−J, where U
and Jare the effective on-site Coulomb and exchange para-
meters, respectively. The value of U
eff
for the Co atoms used
was 3.32 eV.
51
In the present study, the antiferromagnetic con-
figuration was found to be the lower energy one. The unit-cell
parameters and the atomic positions were fully optimized to
obtain the relaxed structure at selected volumes. The criteria
imposed for the optimization process were that the forces on
the atoms should be smaller than 0.003 eV Å
−1
and the devi-
ations of the stress tensors from a diagonal hydrostatic form
were smaller than 0.1 GPa. In this manner, the simulations
provide a data set of volumes, energies, and pressures (from
the stress tensor). These were fitted with a third-order Birch–
Murnaghan equation of state
52
to obtain theoretical equili-
brium volume, bulk modulus and its pressure derivative.
Lattice-dynamic calculations of the phonon modes were
carried out at the zone centre (Γpoint) of the BZ with the
direct force-constant approach.
53
These calculations give the
frequencies of the normal modes, their symmetries, and their
polarization vectors. This allows the identification of the irre-
ducible representations and the character of the phonon
modes at the Γ-point. A 2 × 2 × 2 supercell was used to obtain
the phonon dispersion and to check the dynamical stability of
the analysed structures. Mechanical properties were studied
from the calculations of the elastic constants using the stress–
strain methodology implemented in VASP using the Le Page
54
methodology. From these elastic constants, we derived the
different elastic moduli and analyzed the mechanical stability.
III. Results and discussion
A. XRD experiments
We performed two HP XRD experiments, one up to 20.60(5)
GPa (experiment 1) and the another (experiment 2) only up to
4.60(5) GPa. In the first experiment we observed the onset of a
phase transition at 2.90(5) GPa. The second experiment was
performed only up to pressures slightly above the transition
pressure to confirm the results of the first experiment and to
collect more data for the EOS determination of the low-
pressure phase. In experiment 2, the transition pressure was
2.85(5) GPa, which agrees with the first experiment. In Fig. 2,
we present a selection of HP XRD patterns recorded in the first
experiment at different pressure conditions. In the figure, it
can be seen that XRD patterns at 0.30(5) and 2.20(5) GPa agree
well with a crystal structure (low-pressure, LP) isomorphic to
that of the farringtonite-type phase. Rietveld refinement plots
are included to support this statement. The goodness of fit
parameters for the experiment performed at 0.30(5) GPa are R
p
= 2.85%, R
wp
= 3.62%, and χ
2
= 1.24. Similar values were
obtained for the low-pressure phase in both experiments at
different pressures confirming the correctness of the structural
model. The refined crystal structure parameters at 0.15(5) GPa
are given in Table 1, and the corresponding CIF file can be
downloaded from the Cambridge Crystallographic Data Centre
(CCDC), using the deposition number 2354014. The present
observed crystal structure is very similar to the structure of
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Co
3
P
2
O
8
reported at ambient conditions
24,30
with only small
changes in lattice parameters and atomic positions. Relevant
bond distances in the structure are given in Table 1. They
agree with those previously reported in the literature
24,30
at
ambient conditions.
In experiments 1 and 2, we detected the appearance of extra
diffraction peaks at 2.90(5) GPa and 2.85(5) GPa, respectively.
This can be seen in Fig. 2 which depicts the data from experi-
ment 1. Some of the more prominent additional peaks are
marked with red asterisks in Fig. 2. In the same figure at 2.90
(5) GPa we identify with black asterisks peaks that undoubtedly
belong to the low-pressure phase. We associated the appear-
ance of extra peaks as the onset of a structural phase tran-
sition. As pressure was increased, we found that the peaks of
the low-pressure phase became weaker, and the peaks of the
high-pressure phase became stronger. However, there are
peaks of the low-pressure phase that can be identified up to
10.3(1) GPa. They are denoted by black asterisk in Fig. 2. The
peaks of the low-pressure phase completely disappear at 10.5
(1) GPa supporting the completion of the phase transition. We
believe the coexistence of the low-pressure and high-pressure
phase might be connected to a kinetic barrier similar to that
observed in the related phosphate, LiNiPO
4
.
55
As we will
describe below, the reported phase transition is reconstructive
involving the modification of primary chemical bonds.
Consequently, the phase transformation needs to overcome an
energy barrier (the kinetic barrier) which makes the transition
slow and favours the coexistence of the low- and high-pressure
phases. The phase coexistence is not due to non-hydrostatic
conditions or pressure gradients because in this pressure
range the conditions are quasi-hydrostatic.
39
This argument is
consistent with the fact that there is no significant peak broad-
ening in the XRD experiment under HP.
For the structural assignation of the HP phase, we first
indexed the XRD pattern measured at 10.5(1) GPa using
DICVOL
56
and then assigned the symmetry. It is found that
the space group P2
1
/cgives the best figure of merit, and that
the unit-cell parameters determined after indexation
resembled those of the metastable olivine-type Co
3
P
2
O
8
reported by Berthet et al. which is isostructural to chopinite.
27
Then, we refined the atomic positions by Rietveld refinement
method using atomic positions reported by Berthet et al.
27
as
initial position coordinates. Fig. 2 shows the Rietveld refine-
ment plot for the XRD data obtained at 10.5(1) GPa. The
refined structure gives small residuals and goodness-of-fit
parameters, R
p
= 2.85%, R
wp
= 4.44%, and χ
2
= 1.57. The unit-
cell parameters and atomic position determined at 10.5(1) GPa
are summarized in Table 2. This table also include bond dis-
tances which are similar to those reported at ambient pressure
in the literature.
27
The CIF file can be downloaded from CCDC
under deposition number 2355641. The crystal structure of the
HP phase is shown in Fig. 1b. In the structure of HP phase, all
the Co atoms are in octahedral coordination. The relevant
bond distances are given in Table 2. XRD patterns measured
upon further compression can be fitted with the same struc-
tural model as shown in Fig. 2 for the measurement at 20.5(2)
Fig. 2 Rietveld refinement profiles of powder XRD patterns measured
in Co
3
P
2
O
8
at selected pressures shown in the figure. Pressures are
given in the figure with the uncertainties given between brackets. The
black circles are the experimental data, and the red (black) lines are the
refinements (residuals). Vertical black (red) ticks are the calculated posi-
tions of the reflections of the low(high)-pressure phase. The Cu peaks
used to measure pressure are indicated. Black (red) asterisks are the
peaks of the low(high)-pressure phase discussed in the text.
Table 1 Experimental unit-cell parameters, atomic positions, and
bond-distances (in Å) of the low-pressure phase of Co
3
P
2
O
8
at 0.15(5)
GPa. CCDC 2354014
a= 5.0635(1) Å, b= 8.3594(2) Å, c= 8.7865(2) Å, and β= 121.05(2)°
Atom Site xy z
Co1 4e 0.5169(4) 0.8589(4) 0.6119(10)
Co2 2b 0.5 0 0
P 4e 0.8385(2) 0.1940(2) 0.8006(2)
O1 4e 0.7687(9) 0.1424(9) −0.0549(9)
O2 4e 0.1882(9) 0.1956(4) 0.8739(4)
O3 4e 0.6875(4) 0.3637(4) 0.7419(4)
O4 4e 0.6888(4) 0.0775 (5) 0.6410(4)
P–O2 = 1.542(5), P–O1 = 1.545(9), P–O4 = 1.547(4), and P–O3 = 1.567(4).
Co1–O2 = 1.976(6), Co1–O4 = 1.979(9), Co1–O4 = 1.983(5), Co1–O3 =
2.023(9), and Co1–O1 = 2.207(6). Co2–O2 = 2.143(4) (x2), Co2–O1 =
2.044(8) (x2), and Co2–O3 = 2.153(4) (x2).
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GPa. After decompression, at the lowest pressure, the recovered
sample contained both the HP and LP phases, supporting the
hypothesis that the transition might be hindered by a kinetic
barrier.
57
The observed phase transition involves a structural reorgan-
ization and an abrupt decrease of the unit-cell volume, as dis-
cussed below. These observations suggest the phase transform-
ation is a first-order reconstructive phase transition. An impor-
tant structural change in the phase transformation is related
to the coordination number of Co atoms. The low-pressure
phase the Co1O
6
octahedra and Co2O
5
units share edges to
form a zig-zag –Co1O
5
–Co2O
6
–Co1O
5
–chain. In contrast, the
high-pressure phase has a closely similar framework formed
by edge sharing Co2O
6
–Co1O
6
–Co2O
6
octahedral chains. The
presence of penta-coordinated Co ions near a vacant coordi-
nation site is conducive to the formation of additional Co–O
bonds at relatively low compression, thereby favouring the
observed phase transition. The high-pressure phase is an
olivine-type structure with ordered vacancies. The structure
can be seen as a monoclinic distortion of orthorhombic
Co
2
Li
2
P
2
O
8
(space group Pnma),
58
where the two Li atoms are
substituted by one cobalt atom and a vacancy, forming [Co
3
□]
P
2
O
8
. The presence of ordered vacancies favors tilting of CoO
6
octahedra which triggers the reduction of symmetry of the
crystal structure. In fact, the HP chopinite-type structure can
be obtained by a group–subgroup transformation from the
structure of Co
2
Li
2
P
2
O
8
by letting the βangle deviate from 90°.
The occurrence of the observed phase transition is consistent
with the fact that chopinite has been found as a HP polymorph
of Mg-dominant farringtonite in meteoritic minerals found in
Antarctica.
23
From the Rietveld refinements of the XRD patterns
acquired on sample compression, we determined the pressure
dependence of the unit-cell parameters. The pressure depen-
dence of the unit-cell parameters is presented in Fig. 3. In the
figure, we also show the results from density functional theory
(DFT) calculations for comparison. DFT calculated results
agree quite well with experiments for the LP phase. For the HP
phase the calculated data show slightly underestimated lattice
parameters a,b, and cwhile a slightly overestimated angle β.
As a consequence, the volume is underestimated by 3% in DFT
simulations. This difference is typical of DFT calculations per-
formed within the GGA approximation as it is related to the
approximation used to describe the exchange–correlation
energy. Such differences are also observed in the elastic con-
stants. The causes for such differences have historically been
discussed in the literature.
59,60
In the LP phase the parameter
afollows a non-linear behaviour, decreasing up to around 3
GPa (i.e., the transition pressure) and then increasing with
pressure. The other two parameters have a similar compressi-
bility, and the angle βincreases with pressure enhancing the
monoclinic distortion of the structure. In the HP phase, a,b,
and chave a similar compressibility and βincreases with
pressure. Regarding the pressure dependence of the volume,
in both phases we found that it can be described by a third-
order Birch–Murnaghan equation of state (BM EOS).
52
Table 2 Experimental unit-cell parameters, atomic positions, and
bond-distances (in Å) of the HP phase of Co
3
P
2
O
8
at 10.50(5) GPa.
CCDC 2355641
a= 5.7858(1) Å, b= 4.7475(2) Å, c= 9.9344(2) Å, and β= 91.72(2)°
Atom Site xyz
Co1 2b 0.5 0 0
Co2 4e 0.7616(9) 0.5143(9) 0.2279(9)
P 4e 0.7471(9) 0.0753(9) 0.4058(9)
O1 4e 0.7316(15) 0.7481(15) 0.4013(15)
O2 4e 0.7493(15) 0.3057(15) 0.0487(15)
O3 4e −0.0544(15) 0.1932(15) 0.3233(15)
O4 4e 0.5296(15) 0.2281(15) 0.3377(15)
P–O2 = 1.528(17), P–O3 = 1.536(13), P–O1 = 1.557(9), and P–O4 = 1.586
(12). Co1–O1 = 2.055(11) (x2), Co1–O4 = 2.077(13) (x2), and Co1–O2 =
2.093(9) (x2). Co2–O3 = 1.975(11), Co2–O2 = 2.037(16), Co2–O4 = 2.056
(11), Co2–O1 = 2.061(16), Co2–O3 = 2.072(12), and Co2–O4 = 2.219(13).
Fig. 3 Pressure dependence of the unit-cell parameters a,b,c, and β
and volume, for both the LP and HP phases of Co
3
P
2
O
8
. Empty circles
(squares) are from experiment 1 (2). The black triangles are the results at
ambient conditions. Black (red) colour is used for the LP (HP) phase. The
solid lines are the results from DFT simulations, and the dashed blue
lines are the equations of state fitted to the experimental data described
in the text. The associated errors are smaller than symbols.
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According to the fits shown in Fig. 3, for the LP phase the
experimental unit-cell volume at zero pressure, bulk modulus,
and its pressure derivative are: V
0
= 319.3(2) Å
3
,B
0
= 75(2) GPa,
and B′
0
= 2.1(3), respectively, while the corresponding values
for the HP phase are: V
0
= 299.4(2) Å, B
0
= 92(2) GPa and B′
0
=
4.2(2). The increase in B
0
and B
0
′after the phase transition
indicates a decrease of the compressibility, which is consistent
with the abrupt decrease of the volume by 6.5%, which makes
the HP phase denser than the LP phase. The discontinuity of
the volume at the phase transition is consistent with a first-
Fig. 4 (a) DFT calculated enthalpy vs. pressure for the low-pressure
(black) and high-pressure (red) phases of Co
3
P
2
O
8
. The vertical blue
dashed line shows the pressure where the enthalpy curves cross inter-
sect. (b) DFT calculated energy vs. volume for the same phases. To
identify phases in (b), we have used the same colour code than in (a).
Table 3 Calculated elastic constants C
ij
(in GPa) for LP Co
3
P
2
O
8
at 0
GPa. The bulk (B), shear (G), and Young (E) moduli, Poisson’s ratio (n),
and B/Gratio are also included. B,G, and Eare given in GPa
C
ij
Property
C
11
184.5 B80.6
C
22
129.3 G34.4
C
33
100.6 E 90.4
C
44
39.0 ν0.313
C
55
30.1 B/G2.34
C
66
32.9
C
12
77.4
C
13
66.7
C
15
−2.8
C
23
40.1
C
25
3.4
C
35
−13.8
C
46
−0.7
Fig. 5 Phonon dispersion of (a) the low-pressure phase of Co
3
P
2
O
8
at
0 GPa and (b) the high-pressure phase of Co
3
P
2
O
8
at 5.5 GPa.
Table 4 Calculated elastic constants C
ij
(in GPa) for HP Co
3
P
2
O
8
at 0
GPa. The bulk (B), shear (G), and Young (E) moduli, Poisson’s ratio (n),
and B/Gratio are also included. B,G, and Eare given in GPa
C
ij
Property
C
11
133.5 B100.7
C
22
202.5 G48.8
C
33
206.1 E 126.1
C
44
59.1 ν0.291
C
55
37.8 B/G2.06
C
66
44.9
C
12
57.1
C
13
66.1
C
15
−8.2
C
23
80.4
C
25
−17.7
C
35
−11.2
C
46
−13.6
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order nature for the transition and consistent with coexistence
of both LP and HP phases in wider pressure range. The bulk
modulus we report for LP Co
3
P
2
O
8
(farringtonite-type) is
similar to that of α-Ca
3
P
2
O
8
(B
0
= 79(2) GPa and B′
0
= 4).
61
The
bulk modulus of HP Co
3
P
2
O
8
(chopinite-type) is similar to that
reported recently for isostructural Mg
3
P
2
O
8
(B
0
= 82(2) GPa and
B′
0
= 4),
62
Sr
3
P
2
O
8
(B
0
= 89(2) GPa and B′
0
= 6.6(3)),
63
and
olivine-type LiNiPO
4
(B
0
= 88(2) GPa and B′
0
= 3.5(5)).
55
Among
related phosphates, only compounds where one of the cations
is potassium are more compressible, such as K
2
Ce(PO
4
)
2
(B
0
=
52(3) GPa and B′
0
= 5.7(6)).
64
This is because of the relative
weakness of K–O bonds which exhibit significant compressi-
bility under pressure.
B. DFT calculations
In Fig. 4 we present the enthalpy versus pressure, and energy
versus volume (inset) for the LP and HP phases of Co
3
P
2
O
8
.
The calculations show that LP farringtonite-type Co
3
P
2
O
8
is
the structure with the lowest energy and lowest enthalpy at 0
GPa. Thus, the farringtonite-type structure is the thermo-
dynamically most favourable structure of Co
3
P
2
O
8
. As pressure
increases, the volume decreases, and beyond 2.1 GPa, the HP
chopinite-type structure becomes the most stable structure for
Co
3
P
2
O
8
. These results support the occurrence of a structural
phase transition at 2.1 GPa in agreement with experiments.
From calculations we determined the EOS parameters for both
phases as: V
0
= 318.4(2) Å
3
,B
0
= 78(2) GPa, and B′
0
= 2.0(3) (for
low-pressure phase) and V
0
= 286.9(2) Å
3
,B
0
= 91(1) GPa, and
B′
0
= 4.2(2) (for high-pressure phase). The values of the bulk
moduli and their pressure derivatives are also consistent with
those extracted from experiments.
Our calculations also support the mechanical and dynami-
cal stability of the two structures. Tables 3 and 4 show the cal-
culated elastic constants for both structures. All the eigen-
Table 5 Calculated optical phonon frequencies ω(in cm
−1
) at the Γpoint for the low-pressure phase of Co
3
P
2
O
8
at 0 GPa and the high-pressure
phase at 5.5 GPa
Low pressure phase High pressure phase
Raman IR Raman IR
Mode ωMode ωMode ωMode ω
B
g
95.4 A
u
83.6 B
g
111.2 A
u
90.9
A
g
102.8 B
u
111.7 A
g
113.5 B
u
126.9
A
g
114.2 A
u
120.0 B
g
143.5 B
u
149.1
A
g
145.1 B
u
132.7 A
g
146.4 A
u
150.6
B
g
152.0 A
u
148.9 A
g
159.9 A
u
168.6
A
g
157.6 B
u
153.9 B
g
181.1 B
u
188.8
B
g
160.4 A
u
180.1 B
g
193.9 A
u
191.9
A
g
201.4 A
u
186.4 A
g
200.9 A
u
203.6
B
g
202.2 B
u
188.8 A
g
235.5 B
u
206.1
B
g
216.5 B
u
194.5 B
g
244.4 A
u
213.6
A
g
234.9 A
u
200.0 A
g
254.1 B
u
240.4
B
g
235.9 A
u
219.7 B
g
265.9 B
u
253.3
A
g
267.1 B
u
234.2 A
g
291.8 A
u
253.7
A
g
287.8 A
u
250.7 A
g
309.5 A
u
285.0
B
g
294.9 B
u
254.2 B
g
311.9 B
u
292.1
B
g
312.5 A
u
272.3 B
g
333.6 A
u
343.5
B
g
321.9 B
u
286.7 A
g
334.2 B
u
344.9
A
g
332.7 A
u
316.9 B
g
349.9 A
u
353.4
B
g
421.8 B
u
316.9 A
g
388.9 A
u
356.0
A
g
431.4 B
u
329.3 B
g
397.8 B
u
358.0
A
g
461.5 A
u
332.6 A
g
477.3 B
u
380.5
B
g
474.7 B
u
427.7 B
g
491.0 A
u
433.7
B
g
528.6 A
u
430.7 A
g
525.8 B
u
442.4
A
g
532.4 A
u
485.9 B
g
534.2 A
u
485.8
A
g
572.1 B
u
493.7 A
g
583.7 B
u
489.2
B
g
584.0 B
u
529.0 B
g
595.2 A
u
517.6
A
g
584.3 A
u
538.9 A
g
612.2 B
u
532.2
B
g
594.6 A
u
564.8 B
g
626.2 B
u
553.7
A
g
919.6 B
u
572.4 B
g
885.1 A
u
565.8
B
g
928.8 B
u
598.9 A
g
893.2 A
u
595.4
A
g
972.6 A
u
609.1 B
g
934.2 B
u
609.2
A
g
989.7 A
u
916.1 A
g
940.4 B
u
881.6
B
g
1004.5 B
u
922.3 B
g
1025.5 A
u
891.4
A
g
1032.1 A
u
955.4 A
g
1027.2 B
u
933.7
B
g
1041.3 B
u
973.4 A
g
1041.2 A
u
952.4
B
g
1079.9 B
u
988.1 B
g
1048.0 B
u
1031.0
A
u
998.2 A
u
1033.9
B
u
1014.3 B
u
1050.3
A
u
1029.4 A
u
1114.1
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values of both elastic tensors are positive indicating elastic
stability of both phases. The obtained constants also fulfil the
Born criteria
65
and confirm that the structures are mechani-
cally stable. From the elastic constants, we have calculated the
elastic moduli using the average between the values obtained
with the Hill and Reuss approximations.
66
The obtained bulk
moduli for the low-pressure (80.6 GPa) and high-pressure
phases (100.7 GPa) agree with the values obtained from experi-
ments and total-energy calculations. The Young’s modulus, in
the low(high)-pressure phase is 12% (20%) larger than the
bulk modulus, indicating that in both phases the tensile and
compressive stiffness when the force is applied lengthwise is
larger than their resistance to bulk compression. We also
found that in both phases the shear modulus is considerably
smaller than the bulk modulus indicating that shear defor-
mations are favoured over volume contraction, making both
phases of Co
3
P
2
O
8
susceptible to large non-hydrostatic stres-
ses.
67
In addition, according to the B/Gratio (>1.75), we can
postulate that the two phases of Co
3
P
2
O
8
are ductile.
68
The
values of the Poisson’s ratio (ν) are also consistent with this
conclusion.
69
To conclude, we present the results of our phonon calcu-
lations. The calculated phonon dispersion of each phase is
shown in Fig. 5. The phonon dispersions indicate that there
are no imaginary phonon branches in the structures examined
in this study. This provides evidence for the dynamic stability
of the two phases of Co
3
P
2
O
8
. We have also calculated the
phonon frequencies and the symmetries for the Raman-active
and infrared (IR)-active modes. The results are summarized in
Table 5. Since there are no Raman or IR experiments reported
for the studied materials, our results can be used as a guide
for future experiments. Using group theory, the following 78
vibrational modes are predicted for both phases at the Γpoint
of the BZ; Γ= 18A
g
+ 18B
g
+ 21A
u
+ 21B
u
. Of these modes, one
A
u
mode and two B
u
modes are the acoustic modes. All of the
remaining modes are optical modes. Out of the modes
present, 36 can be observed in Raman spectroscopy (18A
g
+
18B
g
), and 39 can be observed in IR spectroscopy (20A
u
+
19B
u
). In the low-pressure phase, the distribution of Raman
modes resembles the Raman spectra of Mg
3
P
2
O
869
and
Ca
3
P
2
O
8
.
70
A principal characteristic of the Raman and IR
spectra of the low-pressure phase is the phonon gap from 600
to 915 cm
−1
. This is related to the fact that modes above
915 cm
−1
are associated to internal vibrations of the PO
4
tetra-
hedron. The rest of the modes correspond mainly to lattice
vibrations involving Co atoms and PO
4
tetrahedra moving as
rigid units. Among the internal modes the A
g
modes with fre-
quencies of 987.7 and 1032.1 cm
−1
are the symmetric and
asymmetric stretching modes of the PO
4
tetrahedron. Note
that their frequencies are nearly the same as those observed
for the equivalent modes in Mg
3
P
2
O
8
, which are 987 and
1029 cm
−1
respectively.
69
This is a consequence of the fact that
the PO
4
tetrahedron is nearly identical in orthophosphates.
The Raman and IR spectra of the HP phase are qualitatively
similar to those of the LP phase. The main difference is the
decrease of the phonon gap which in the HP phase is between
625 and 880 cm
−1
. We also notice a decrease of the frequency
of the internal stretching modes, which in the HP phase are
940.4 and 1027.2 cm
−1
. Similar decrease in frequency of these
modes is found when comparing the Raman spectra of farring-
tonite-type and chopinite-type Mg
3
P
2
O
8
.
69
Such decrease in
the frequency of phonon modes is related to the distortion of
the PO
4
tetrahedron.
IV. Conclusions
Through high-pressure X-ray diffraction experiments and
density-functional theory calculations it has been concluded
that Co
3
P
2
O
8
experiences a phase transition beyond 2.9 GPa.
The crystal structure of the new high-pressure phase has been
determined. The occurrence of phase transition is connected
to the presence of CoO
5
trigonal bipyramids in the low-
pressure phase and the formation of an additional Co–O bond
induced by compression. We have also established the
pressure dependence of the unit-cell parameters and a room-
temperature equation of state for each phase. The obtained
bulk moduli are discussed in comparison with related ortho-
phosphates. Calculations also give the details about the elastic
constants and moduli, and phonon frequencies, they may be
important roles for using Co
3
P
2
O
8
in various applications.
Author contributions
Robin Turnbull, Josu Sánchez Martín, Akun Liang, Daniel
Díaz-Anichtchenko, Catalin Popescu, K. Sandeep Rao,
S. Nagabhusan Achary, and Alfonso Muñoz: investigation,
writing –review & editing. Vinod Panchal: formal analysis,
writing –review & editing. Daniel Errandonea: conceptualiz-
ation, formal analysis, funding acquisition, investigation,
project administration, writing –original draft, writing –
review & editing.
Data availability
All relevant data are available from the corresponding author
upon reasonable request.
Conflicts of interest
The authors declare that they have no known competing finan-
cial interests or personal relationships that could have
appeared to influence the work reported in this paper.
Acknowledgements
The authors thank the financial support from the Spanish
Ministerio de Ciencia, Innovación y Universidades, MCIU,
(https://doi.org/10.13039/501100011033) under Projects PID2022-
Paper Dalton Transactions
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138076NB-C41/44, and RED2022-134388-T. They also acknowl-
edge the financial support of Generalitat Valenciana through
grants PROMETEO CIPROM/2021/075 and MFA/2022/007. R. T.
acknowledges funding from the Generalitat Valenciana for
Postdoctoral Fellowship no. CIAPOS/2021/20. J. S.-M. acknowl-
edges the Spanish MCIU for the PRE2020-092198 fellowship.
C. P. recognizes the financial support from the Spanish
Ministerio de Ciencia e Innovacion through project PID2021-
125927NB-C21. This study forms part of the Advanced
Materials program and is supported by MCIU with funding
from European Union Next Generation EU (PRTR-C17.I1) and
by the Generalitat Valenciana. The authors thanks ALBA for
providing beamtime under experiment no. 2022085940.
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