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First PAC experiments in MAX-phases

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D. Jürgens
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Michael Uhrmacher at University of Göttingen
Michael Uhrmacher
H. Hofsäss
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Jens Röder at CERN
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Abstract and Figures

MAX-phases are hexagonal ternary carbides and nitrides with the general formula: M n + 1 AX n and n = 1 to 3. 111In was implanted into the two MAX compounds Ti2InC and Zr2InC. Based on the general knowledge of previous 111In implantations one expects to find the probes on the indium lattice-site in these compounds. First experiments on the annealing behaviour and the thermal stability of the indium-containing MAX-phases are reported. The observed EFGs are interpreted and first PAC-measurements under compressive stress are shown.
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Hyperfine Interact (2007) 178:23–30
DOI 10.1007/s10751-008-9651-7
First PAC experiments in MAX-phases
D. Jürgens ·M. Uhrmacher ·H. Hofsäss ·J. Röder ·
P. Wodniecki ·A. Kulinska ·M. Barsoum
Published online: 6 August 2008
© The Author(s) 2008
Abstract MAX-phases are hexagonal ternary carbides and nitrides with the general
formula: Mn+1AXnand n=1to3.111In was implanted into the two MAX compounds
Ti2InC and Zr2InC. Based on the general knowledge of previous 111In implantations
one expects to find the probes on the indium lattice-site in these compounds. First
experiments on the annealing behaviour and the thermal stability of the indium-
containing MAX-phases are reported. The observed EFGs are interpreted and first
PAC-measurements under compressive stress are shown.
Keywords Perturbed angular correlation (PAC) ·MAX-phase ·Ti2InC ·Zr2InC ·
Static pressure
1 Introduction
MAX-phases are layered, hexagonal ternary carbides and nitrides (general formula:
Mn+1AXnwhere nvaries from 1 to 3). Mstands for an early transition metal, A
for an A-group (mostly IIIA and IVA) element and Xrepresents either C and/or
N. They belong to space group D4
6h,P6
3/mmc, with two formula units per unit cell
(see Fig. 1). X-ions sit in the centre of an M-octahedron. The 211 MAX-phases
D. Jürgens ·M. Uhrmacher (B)·H. Hofsäss
II. Physikalisches Institut, Universität Göttingen,
Friedrich-Hund-Platz 1, 37077 Göttingen, Germany
e-mail: muhrmac@gwdg.de
J. Röder
Institut für Physikalische Chemie, TU Braunschweig, Braunschweig, Germany
P. Wodniecki ·A. Kulinska
IFJPAN, 31-342 Krakow, Poland
M. Barsoum
Dep. Mat. Sci. +Eng., Drexel University, Philadelphia, PA 19104, USA
24 D. Jürgens et al.
Fig. 1 Structure of M2AX1
(=211): Mgray, Awhite, X
black
(n= 1) show in the direction of the c-axis the layer-sequence AMXMA..., the
312 phases (n= 2) show AMXMXMA... and the 413 phases (n=3)comewith
AMXMXMXMA....
These compounds combine some of the best properties of metals and ceramics.
Like metals, they are electrically and thermally conductive, most readily machinable,
not susceptible to thermal shock, plastic at high temperatures, and exceptionally
damage tolerant. Like ceramics, they are elastically rigid, lightweight, and maintain
their strengths to high temperatures. The ternaries Ti3SiC2and Ti2AlC are creep,
fatigue and oxidation resistant [1]. At present the explanation of this extraordinary
behavior is assumed to be in the microstructure of the layered material: kinking
bands and delamination seem to play a central role [2].
Using perturbed γγ-angular correlations (PAC) with 111In probe nuclei, changes
of MAX-phase properties during elastic deformations can be observed on an atomic
scale. PAC experiments in complex compounds with different crystallographic sites
often suffer from the problem to find the position of the probes. In the two MAX
compounds Ti2InC and Zr2InC the probe 111In is a constituting element of the
compounds. Therefore, one expects to find the probes on the indium lattice-site,
which should establish a PAC fingerprint, typically for the In-site in MAX phases,
or more general for the A-site in the MAX compounds. In complete analogy, the
M-site fingerprint can be discovered using the 181Hf probe. Such studies will provide
First PAC experiments in MAX-phases 25
Fig. 2 Pressed Ti2InC—seen
with an electron raster
microscope. The typical
nanolaminate structure of a
MAX-phase clearly shows up
the key information to investigate by PAC the microstructure of the full class of
MAX phases (about 50 compounds) which do not necessarily contain In-ions on
the A-site or Hf-ions on the M-site of the structure. Knowing the “fingerprints” the
technical important MAX-phases can be investigated.
2 Experimental details
The Ti2InC and Zr2InC samples have been provided by M. Barsoum. The fabrication
is described in Ref. [3]: Elemental powders of Zr, Ti, In and graphite have been
mixed in the proper stoichiometric ratios and cold pressed at 630 MPa. The resulting
cylindrical pellets were sealed in borosilicate glass tubes under vacuum, placed in a
hot isostatic press (HIP), heated at 20◦C/min to 973 K and held at that temperature
for 30 min, before Ar was introduced to the chamber. The HIP was then heated at
the same rate to 1573 K, where it was held for 7 h before cooling. The pressure at
1573 K was ∼50 MPa. Finally the glass was removed mechanically. Predominantly
single phase, fully dense samples were obtained. They were cut by a diamond saw
into 20 mm2large slices of about 1 mm thickness.
About 1012 of 111In+ions were implanted at 400 keV into such samples, using the
Göttingen implanter IONAS [4]. The radiation damage after the implantation was
annealed out by heating the samples above Ta= 700 K in vacuum, which caused
some problems (see Section 3.1). PAC-spectra were taken at different measuring
temperatures Tm, with the help of a standard setup of four NaI-detectors in 90◦
geometry. Details on the data analysis can be found in [5]. The static pressure was
applied by placing a sample between two DURAL-pistons, which were screwed
against each other (Fig. 2). A pressure of about 1 GPa could be estimated from the
“engrammes” made by the sample in the Al.
26 D. Jürgens et al.
3Results
3.1 Indium loss
In first studies it was reported, that Ti2InC evaporates indium, when heated in
vacuum at 1,173 K for ∼2 h. XRD showed the emergence of peaks corresponding
to TiCx. Also a weight loss was found. It was assumed that Ti2InC dissociates
peritectically into the A-group element and the MX2phase [3]. Some light on the
final dissociation product came from FPLAPW-calculations of ordered titanium
carbide (Ti2C)-phases. The trigonal phase was found to be more stable than the cubic
one, but all calculated Ti2C phases were found to be stable against segregation into
TiC and metallic Ti [6]. The observation of In-whisker formation on Zr2InC samples
can be seen in the same context: The samples contained some unreacted indium
in the grain boundaries. The In-content, determined from differential scanning
calorimetric analysis, was 4 vol. %. The majority of the grains ranged in the size
between 3–5 μm[7].
Heating Ti2InC for 1 day at 473 K and for 1 day at 573 K in vacuum (PAC-
tempering sequence) caused a loss of about of 30% of the implanted 111In probes.
Less pronounced was this loss for Zr2InC under similar conditions, and for Ti2InC
it could be lowered by a vacuum annealing for only 10 min at 873 K.
3.2 The MAX-phase Ti2InC
PAC spectra after the implantation (Fig. 3, top) show the need for an annealing
step, although some 111In-loss can be expected (Section 3.1). Afterwards a fraction
fIn ∼50% of the probes were found at the substitutional site in In-metal precipitates,
identified by the well known frequency of 17 MHz with η=0[8]. When heating
the sample above the melting point of indium (430 K), the precipitates transform
into liquids, which have no EFG. Consequently, a PAC measurement at Tm= 436 K
shows now the fraction fIn ∼50% with νQ= 0 MHz. This process is reversible and
proves the existence of In-precipitates, un-reacted indium or decomposed MAX-
phase. The rest of the probes has a well defined EFG, fitted with the parameters
νQ= 290 MHz and η= 0. Spectra are given in Fig. 3, the temperature and annealing
conditions are given in the same figure. XRD analyses before and after the PAC
cycle showed the compound still intact and gave no hints of disintegration phases.
Therefore, this high frequency is attributed to probes at the In-lattice site (A-site) of
Ti2InC.
3.3 The MAX-phase Zr2InC
The necessary vacuum annealing step after the 111In-implantation caused—as
expected—also in this MAX-compound a loss of indium. The contaminated con-
tainer/envelope was removed and also this compound showed 65% of the probes
with the typical parameters of the substitutional site in In-metal (νQ=17MHz,
η= 0). Above the indium melting point we found fIn = 65% with νQ= 0 MHz, a clear
proof, that the 111In is located in metallic indium. The rest of the probes showed two
First PAC experiments in MAX-phases 27
Fig. 3 PAC-spectra and their Fourier transforms for Ti2InC
well defined EFGs of similar magnitude, fitted with the parameters νQ1 = 348 MHz,
νQ2 = 328 MHz and η1=η2= 0 (see Table 1). The PAC-spectra are given in Fig. 4.
The temperature and annealing conditions are included in the same figure. Similarly
to the case of Ti2InC we attribute this high frequency to probes on the In-lattice site
of Zr2InC.
28 D. Jürgens et al.
Table 1 PAC parameters of the A-site EFG (measured at RT) in different MAX-phases: νQand
ηdescribe the strength and symmetry of the EFG, δis the distribution-width around νQand the
fraction gives the percentage of probes on this A-site
Compound νQ[MHz] ηδ[MHz] Fraction [%]
Ti2InC 290 (3) 0 6 (1) 43–65
Zr2InC 348 (3) 0 2 (0.3) 25
328 (9) 0 5 (1) 12.5
Ti3SiC2348 (3) 0 2 (0.3) 68
Fig. 4 PAC-Spectra and their Fourier transforms for Zr2InC
3.4 Compressive stress on Ti2InC
After all annealing steps, which prepared a maximum of the A-site EFG (Fig. 5,
upper part), the Ti2InC sample of Fig. 3was placed between two Al-pistons. The
sample’s position was flat in the detector plane. Under compressive stress, the PAC
First PAC experiments in MAX-phases 29
Fig. 5 Ti2InC well annealed (upper panel), under pressure (lower panel). The line shows the part of
the R(t)-function, which is caused by probes in metallic indium
spectrum (Fig. 5, lower part) showed a clear texture with the EFG pointing into the
detector plane, i.e. vertically to the direction of the pressure.
4 Discussion
The idea to find a “fingerprint EFG” of the A-site in MAX phases seems to work.
In Table 1the observed high frequencies are collected, including a first result on
Ti3SiC2, probably the technologically most important MAX-phase, which is stable
to high temperatures and contains no indium in the structure. In all three of them
we observed high EFGs in the range between 290 and 350 MHz. As demonstrated
in the oxides [9] different compounds of the same lattice structures (as an example
bixbyite class: In2O3,Y
2O3, ...) show similar EFGs for a probe on a specific site in that
structure. Despite the problematic question of a disintegration of the In-containing
MAX phases, we assume that these new high EFGs are typical for probes on the
A-site in the MAX phases.
All three compounds show a different result, therefore carbon-precipitates can be
excluded, as they should give the same frequency in all three cases. Nevertheless,
30 D. Jürgens et al.
the identical frequency νQ= 348 MHz is observed in two different compounds. The
strength of the observed EFGs is similar to impurity-In pairs in semiconductors
[10]. We propose, that the 111In probe—sitting on the A-site—catches an interstitial
carbon atom. The small distance within this pair causes the high EFG. A trapping
in slightly different geometries might explain, why in Zr2InC two very similar high
EFGs are observed. Unfortunately, this hypothesis can not yet be proven, but has to
be solved in the future.
One of the extraordinary features of the MAX-phases is the elastic behavior
of these ceramics. Our first simple compression experiment shows, that the PAC
technique is sensitive to this question. Having the probe on a well known site, we will
be able to learn more about the microscopic changes during application and release
of pressure in these layered structures.
Acknowledgement A part of the study was supported by the BMBF under contract 05KK7MG1.
Open Access This article is distributed under the terms of the Creative Commons Attribution
Noncommercial License which permits any noncommercial use, distribution, and reproduction in
any medium, provided the original author(s) and source are credited.
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... For some MAX phases this question can easily be answered by choosing the compounds which have indium as a constituent. Owing to that consideration, previous experiments were performed for the key-compounds Ti 2 InC and Zr 2 InC [10] since the A-element is chemically identical to the 111 In atoms. One expects that the probes occupy the In-site or more generally the A-site. ...
... Simply speaking, one gets a ''fingerprint" of a certain lattice site. We found in [10] that these phases show, after long annealings at high temperatures, axially symmetric EFGs with quadupole coupling con- Using 111 In in other 211-phases which do not contain indium as a constituent, one expects an EFG in the same range of about 300 MHz assuming 111 In atoms at A-sites. The purpose of this work is to characterize the A-site EFG parameters for 111 In incorporated in the compounds Ti 2 AlN and Cr 2 GeC and to study the annealing behavior after implantation of In probe atoms. ...
... Above 1000 K more than 25% of the still present 111 In probes were lost from the samples, reaching a maximum value of 45-65% at 1273 K depending on the annealing duration. Previous studies of indium containing MAX phases have shown that the loss occured by the precipitation of excessive indium [10]. ...
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View
... Previous work on MAX phases, using PAC with an 111 In-111 Cd probe, revealed the existence of A site specific, strong, and axially symmetric EFGs in In, Al, Ge, and As containing 211-MAX phases [22][23][24]. Based on those results, it is now possible to investigate the deformation of polycrystalline MAX phases by an experiment, in which PAC spectra, measured under uniaxial load and after removing the load, are compared. ...
... Slight deviations from an ideal EFG give rise to an attenuation of R(t). The assumption of a distribution in only V zz [35] leads to the widely used expression for the perturbation function of a single fraction of probe surroundings with the distribution width δ, the amplitudes s 2n and coefficients g n depending solely on η [33,34], and the anisotropy of the γ-γ-cascade A 22 . For a = 1, Lorentzian, and for a = 2, Gaussian distributions are obtained. ...
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... This kind of cross-section has previously been observed in the e.g. MAXphases [48] and in the W 2 B 5 phase [2] and is expected for a nanolaminated material where the bond strengths between the nanolaminated slabs are relatively weak. EDS analyses of the grains showing this behaviour confirmed the 1:2 stoichiometry of the Al:M ratio in the bulk sample. ...
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  • Feb 2007
  • PHYSICA B
Nearly all elements can be oxidized and develop oxides, often with different oxygen contents and in different crystalline phases. Applying the classical perturbed angular correlation (PAC)-probes 111In/111Cd or 181Hf/181Ta, the probes are usually found on unperturbed cation lattice sites surrounded by oxygen atoms. Using different oxides of the same structure or comparing different crystal classes the position of the oxygen neighbours near the probe can be varied in a wide range. This allows testing theoretical concepts of electric field gradient (EFG) calculation.In general, the melting point of an oxide is very high, and the PAC experiments span a huge temperature range from Tm=10 to 1700K. Two temperature regions are known, where 111In/111Cd probes show dynamic hyperfine-interactions, which occur when the EFG changes direction or strength during the lifetime of the probe. At low temperatures the electron capture “after-effect” is observed, caused by a low availability of charge carriers in semiconducting or isolating oxides. At very high temperatures intrinsic defects or mobile atoms in ternary oxides move so fast, that undamped perturbation functions arise.Realizing the big impact of STM and AFM to the surface science, a probing technique like PAC for the next neighbours inside a sample seems to be attractive. In the past, numerous discussions asked whether the inserted PAC-probes are really spies—only observers—or if they actually change that neighbourhood, that they are supposed to analyse. Distortions in oxides are discussed.
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Electronic structure and energetics of ordered titanium carbides of composition Ti2C
Article
  • Apr 2002
The equilibrium geometries, formation energies, band structures, densities of states and charge densities of ordered titanium carbide phases of composition Ti2C - cubic Fd3m-Ti2C and trigonal Rm-Ti2C - were calculated self-consistently by means of the full-potential linearized augmented-plane-wave method. The trigonal phase was found to be more stable than the cubic phase by 11.6 kJ/(mole of atoms) because it enables more efficient d-d bonding between Ti d states. The cubic phase is stabilized by the relaxation of the Ti atoms next to the vacancies towards their nearest-neighbour C atoms. In agreement with experiment, the maximum of the stabilizing relaxation energy (2.8 kJ/(mole of atoms)) is found for a relaxation of 0.04 Å. The formation energies are in good agreement with the available experimental values for TiC and Fd3m-Ti2C. Calculations were also performed for two tetragonal phases of composition Ti2X found experimentally for the nitride but not for the carbide. All calculated ordered Ti2C phases are found to be stable against segregation into TiC and metallic Ti.
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Preparation of ion-implanted and sub-monolayer 111In-tracer layers for perturbed angular correlation analysis
Article
  • Apr 1998
  • NUCL INSTRUM METH B
Some 1012 radioactive 111In-tracer atoms are routinely implanted at 400 keV into different samples to perform Perturbed Angular Correlation (PAC) measurements. The experimental details and the tricks used during the preparation and implantation are summarized. As an alternative to ion-implantation, a method to deposit submonolayer 111In-tracer films into metallic multilayers is described. The different applications and benefits of both techniques are compared.
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Fabrication and electrical and thermal properties of Ti2InC, Hf2InC and (Ti,Hf)2InC
Article
  • Jun 2002
  • J ALLOY COMPD
In this paper we report on the characterization of predominantly single phase, fully dense Ti2InC (Ti1.96InC1.15), Hf2InC (Hf1.94InC1.26) and (Ti,Hf)2InC ((Ti0.47,Hf0.56)2InC1.26) samples produced by reactive hot isostatic pressing of the elemental powders. The a and c lattice parameters in nm, were, respectively: 0.3134; 1.4077 for Ti2InC; 0.322, 1.443 for (Ti,Hf)2InC; and 0.331 and 1.472 for Hf2InC. The heat capacities, thermal expansion coefficients, thermal and electrical conductivities were measured as a function of temperature. These ternaries are good electrical conductors with a resistivity that increases linearly with increasing temperatures. At 0.28 μΩ m, the room temperature resistivity of (Ti,Hf)2InC is higher than the end members (∼0.2 μΩ m), indicating a solid solution scattering effect. In the 300 to 1273 K temperature range the thermal expansion coefficients are: 7.6×10−6 K−1 for Hf2InC, 9.5×10−6 K−1 for Ti2InC, and 8.6×10−6 K−1 for (Ti,Hf)2InC. They are all good conductors of heat (20 to 26 W/m K) with the electronic component of conductivity dominating at all temperatures. Extended exposure of Ti2InC to vacuum (∼10−4 atm) at ∼800°C, results in the selective sublimation of In, and the conversion of Ti2InC to TiCx.
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  • Jan 1998
  • 306
  • M Uhrmacher
Uhrmacher, M., et al.: Nucl. Instrum. Methods B139, 306 (1998)