Article

Discovery of the Ternary Nanolaminated Compound Nb 2 GeC by a Systematic Theoretical-Experimental Approach

Physical Review Letters (Impact Factor: 7.73). 07/2012; 109:035502.

ABSTRACT Since the advent of theoretical materials science some 60 years ago, there has been a drive to predict and design new materials in silicio. Mathematical optimization procedures to determine phase stability can be generally applicable to complex ternary or higher-order materials systems where the phase diagrams of the binary constituents are sufficiently known. Here, we employ a simplex-optimization procedure to predict new compounds in the ternary Nb-Ge-C system. Our theoretical results show that the hypothetical Nb 2 GeC is stable, and excludes all reasonably conceivable competing hypothetical phases. We verify the existence of the Nb 2 GeC phase by thin film synthesis using magnetron sputtering. This hexagonal nanolaminated phase has a and c lattice parameters of $3:24 # A and 12.82 Å . Today's materials science has yielded an unprecedented frequency of new material discoveries. New complex ce-ramics (borides, carbides, nitrides, and oxides) for a wide range of applications are continuously being synthesized. Much of this work, however, has historically been per-formed in a trial-and-error manner, and improved theoreti-cal input in guidance of experimental work is essential. In response to this challenge, the last decade has especially seen a tremendous increase in theoretical predictions of hypothetical novel materials. Traditionally, the vast major-ity of studies calculate only the cohesive energy of the compound itself, which does not give information if the compound is stable relative to any relevant competing phases. This approach yields an unknown local energy minimum in an enormous parameter space, and can very often yield misleading results. A classic example is the prediction of the -C 3 N 4 phase with Si 3 N 4 structure, which was suggested to be stable and harder than diamond [1]. Extensive experiments were performed and some claimed to have synthesized the -C 3 N 4 phase, but it has been presently established that it most likely does not exist [2–4]. A far better approach is to apply exhaustive data-mining methods to predict new crystal structures [5–8]. However, their basic premise is that it should be known that a material of a specified chemical composition does exist, followed by determination of its most likely crystal struc-ture. Such approaches to predict new phases thus do not truly reflect on whether hypothetical compounds can be expected to exist experimentally. Consequently, realistic stability calculations versus relevant competing phases are necessary, but when performed they are normally done ad hoc rather than by a systematic approach. The system-atic optimization approaches that do exist have mainly been applied to simulate temperature dependence and re-action paths in fully known systems (see, e.g., [9,10]). Here, we apply a linear optimization procedure (based on the simplex method) in which all known competing phases as well as hypothetical competing phases based on neighboring and similar systems are included and the relative stability of any hypothetical compound can be calculated relative to the most stable combination of com-peting phases [11,12]. It should be noted here that the method makes a substantial simplification in accounting only for enthalpy terms, not entropy. Nevertheless, our previous benchmarking confirmed that it gives completely accurate results for existing phases in a fairly large set of well-known carbide and nitride systems [12], but the criti-cal test is whether the method also has predictive power. As a model system for these general research questions, we have chosen to study Nb-Ge-C, where no ternary phases apart from Nb 3 GeC (inverse perovskite) [13] have been reported in the peer-reviewed literature. The binary Nb 5 Ge 3 can accommodate a substantial amount of carbon and is thus more appropriately described as Nb 5 Ge 3 C x . In many similar materials systems, there are phases belonging to the class of materials known as M nþ1 AX n phases (n ¼ 1–3, or ''MAX phases''), a group of inher-ently nanolaminated ternary carbides and nitrides (X) of transition metals (M) interleaved with a group 12–16 ele-ment (A) [14–19]. Most M nþ1 AX n phases are M 2 AX phases (originally called ''H phases'') and have been known since the 1960s, while the number of M 3 AX 2 and M 4 AX 3 phases is relatively limited (around a dozen). It is therefore natural to pose the question whether similar phases could exist in the Nb-Ge-C system. This system is also particularly interesting as it would be reasonable to expect superconductivity in a novel Nb nþ1 GeC n phase. Only very few M nþ1 AX n phases are reported to be super-conductors, but those that are mainly tend to be based on the binary superconductor NbC [15]. Furthermore, these

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    ABSTRACT: We report thermal expansion coefficients of the end members and solid-solution compounds in the Cr2(Alx,Ge1−x)C system. All samples studied were essentially phase-pure Cr2AlxGe1−xC except the Cr2GeC sample, which contained a substantial fraction of Cr5Ge3Cx. X-ray diffraction performed in the 25–800 °C temperature range shows that the in-plane thermal expansion remains essentially constant at about 14 ± 1 × 10−6 K−1 irrespective of Al content. The thermal expansion of the c axis decreases monotonically from 17 ± 1 × 10−6 K−1 for Cr2GeC to ∼12 ± 1 × 10−6 K−1 with increasing Al content. At around the Cr2(Al0.75,Ge0.25)C composition, the thermal expansion coefficients along the two directions are equal; a useful property to minimize thermal residual stresses. This study thus demonstrates that a solid-solution approach is a route for tuning a physical property like the thermal expansion. For completeness, we also include a structure description of the Cr5Ge3Cx phase, which has been reported before but is not well documented. Its space group is P63/mcm and its a and c lattice parameters are 7.14 Å and 4.88 Å, respectively. We also measured the thermal expansion coefficients of the Cr5Ge3Cx phase. They are found to be 16.3 × 10−6 K−1 and 28.4 × 10−6 K−1 along the a and c axes, respectively. Thus, the thermal expansion coefficients of Cr5Ge3Cx are highly anisotropic and considerably larger than those of the Cr2(Alx,Ge1−x)C phases.
    Journal of the European Ceramic Society 04/2013; 33(4):897–904. · 2.36 Impact Factor

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