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Abstract
The theory of structural phase transition in CuTi2S4 is proposed. The symmetry of order parameters, thermodynamics and the mechanism of the atomic structure formation of the rhombohedral Cu-Ti-thiospinel have been studied. The critical order parameter inducing the phase transition has been found. Within the Landau theory of phase transitions, it is shown that the phase state may change from the high-symmetry cubic disordered Fd3[combining macron]m phase to the low-symmetry ordered rhombohedral R3[combining macron]m phase as a result of phase transition of the first order close to the second order. It is shown that the rhombohedral structure of CuTi2S4 is formed as a result of the displacements of all types of atoms and the ordering of Cu-atoms (1 : 1 order type in tetrahedral spinel sites), Ti-atoms (1 : 1 : 6 order type in octahedral spinel sites), and S-atoms (1 : 1 : 3 : 3 order type). The Cu- and Ti-atoms form metal nanoclusters which are named a "bunch" of dimers. The "bunch" of dimers in CuTi2S4 is a new type of self-organization of atoms in frustrated spinel-like structures. It is shown that Ti-atoms also form other types of metal nanoclusters: trimers and tetrahedra.
Highlights
Copper-containing thiospinels exhibit a wide variety of physical properties that make them interesting from a scientific point of view.[1,2,3] For example, large negative magnetoresistance is observed in the ferromagnetic thiospinel compound CuCrZrS4.4 The other spinels CuRh2S4 and CuRh2Se4 are superconductors.[5,6,7,8,9] It is interesting to note that CuRh2S4 is the first compound with the pressure-induced superconductor–insulator transition, which occurs between 5.0 and 5.6 GPa.[10]
Using the results of group-theoretical analysis of the phase transitions occurring according to one critical irreducible representation in the group Fd3%m, which satisfy the Lifshitz criterion,[32] we find that the space group R3%m may be generated by threedimensional irrep k11(t7)(1) as well as two six-dimensional irreps k10(t1)(4) and k10(t3)(4) and two four-dimensional irreps k9(t1)(8,2) and k9(t4)(8,2)
For the first time we investigated the order parameter (OP) symmetry, thermodynamics, and structural mechanism of formation of the lowsymmetry CuTi2S4 phase within a unified approach based on the Landau phenomenological theory of phase transitions
Summary
Copper-containing thiospinels exhibit a wide variety of physical properties that make them interesting from a scientific point of view.[1,2,3] For example, large negative magnetoresistance is observed in the ferromagnetic thiospinel compound CuCrZrS4.4 The other spinels CuRh2S4 and CuRh2Se4 are superconductors.[5,6,7,8,9] It is interesting to note that CuRh2S4 is the first compound with the pressure-induced superconductor–insulator transition, which occurs between 5.0 and 5.6 GPa.[10] The iridium thiospinel CuIr2S4 shows a temperature-induced metal–insulator transition with simultaneous charge ordering and spin dimerization transition which is a rare phenomenon in three-dimensional compounds.[11,12,13,14]. It is assumed that such situation is realized in CuTi2S4.16
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