Control of the Valence Instability of Tetravalent Chromium in 1T-CrSe2 by Anion Substitution and Pressure

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We report on the anion substitution and pressure effects on the layered compound 1T-CrSe2 with unusual-valent chromium. Owing to the competition between the orbital degrees of freedom and the valence instability of Cr4+, the systems of Cr(Se1−xSx)2 and Cr(Se1−yTey)2 exhibit several types of phase transition. As well as the parent compound CrSe2, Te-substituted samples and slightly S-substituted samples of x ≤ 0.025 show two phase transitions, which are likely to be related to both the valence change and orbital ordering of Cr4+. The highly S-substituted samples of x > 0.04 show one phase transition, which can be attributed to orbital ordering without a valence change. The intermediate S-substituted samples of \(0.025 < x \lesssim 0.04\) exhibit the above three types of phase transition and, in addition, another type of transition. In contrast, the pressure effect on CrSe2 is qualitatively different from these anion substitution effects, which indicates that these anion substitution effects are not simply explained by a chemical pressure effect but also by the change of the stability in the Cr4+ state.