Structural phase transition in GaMo4S8 by X-ray powder diffraction

Abstract

GaMo4S8 transforms at T, = 45(5) K from a cubic high-temperature modification with ordered defect spinel structure (space group F43m, a = 9.7356(2) Ä at 300 K) to a rhombohedrally distorted low-temperature modification {R3m, = 6.8506(2) Ä, a^h = 60.533(1)° at 8 K). The transition is of first Order, shows a hysteresis of about 4 K, and leaves about 30% of the cubic phase untransformed. Lattice parameters as a function of temperature are reported between 8 K and 300 K, and structure refinements are performed by the Rietveld method from data recorded on a Guinier diffractometer at 8 K and 300 K. The tetrahedral M04 Clusters expand during the phase transjtion at one of their triangular bases, [Mo -MoL = 2.814(3) Ä at r=300K; 2.89(1) Ä at T=SK, while the other intra-cluster distances, [Mo-Moj] = 2.814(3) Ä at 7= 300 K; 2.81(2) Ä at r = 8 K, and inter-cluster distances, [M04-M04] = 4.070(3) Ä at 7 = 300 K; 4.02-4.04(2) Ä at r = 8 K, remain approximately constant.' Permanent address:

Structural phase transition in GaMo4Se8 and AlMo4S8 by X-ray powder diffraction

“…The amount of cubic phase estimates from the ratio of the two scale factors is greater in Π (40%) than in I (10%). The Mo -S(Se) bond distances within the pseudo-cubic Mo 4 S 4 (Se 4 ) units (marked C, in Figure 4 of Frangois et al 1991) tend to increase, and those between these units (see unit marked C 2 in Figure 4 of Francois et al 1991) tend to decrease at the cubic-to-rhombohedral phase transition. Significant structural differences between the HT and LT modifications of both compounds occur mainly with their tetrahedral MO 4 clusters.…”

“…The specific-heat anomalies in these compounds indicated first-order transitions, but that in Al based II was much less pronounced than those in Ga based I and III (Ben Yaich-Aerrache, 1989). For III refined structure data at various temperatures and a study of the structural phase transition were reported by Francois et al (1991) from X-ray powder diffraction. For II non-refined data at room temperature were reported by Vandenberg and Brazen (1975) from X-ray powder diffraction.…”

Assessing exchange-correlation functional performance in the chalcogenide lacunar spinels GaM4Q8 (M=Mo,

“…10 also possible that the structural phase transition is incomplete at low temperature. 7 We also find a minor trend that higher levels of functionals, as well as larger Hubbard-U values, favor larger structural distortions. Since our DFT simulations are performed at 0 K, while experimental characterizations are performed at finite temperature, our results here are more likely to capture the correct ground state structure where thermal expansion effects are small.…”

“…GaV 4 S 8 , GaV 4 S 8 , GaMo 4 S 8 , and GaMo 4 Se 8 , however, undergo symmetry-lowering structural Jahn-Teller (JT) transitions at ≈40 K from F 43m to R3m (space group no. 160), [6][7][8] followed by spontaneous magnetic ordering at a lower Curie temperature T C [Figure 1(d)]. These displacive distortions lead to a unit cell of slightly different volume, lattice parameters, and rhombohedral angle α rh .…”

“…5 After decades of continuous studies on various properties of the lacunar spinel compounds, the mechanism of these phase transitions as well as the proper theoretical approaches to describe various electronic states are still unclear. For instance, while the vanadium and molybdenum compounds can undergo symmetry-lowering structural phase transitions at low temperature, [6][7][8] the niobium and tantalum lacunar spinels remain in the highsymmetry cubic phase over a broad temperature range. 9 One of the possible reasons for this behavior in the family may be attributed to variations in the strength of electron-electron interactions, 6 since electron-correlation effects are expected to be stronger in 3d rather than 5d transition metals.…”