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We report synthesis of polycrystalline samples of the recently discovered spin liquid material Ca10Cr7O28 and present measurements of the ambient and high pressure magnetic susceptibility χ versus temperature T , magnetization M versus magnetic field H at various T , and heat capacity C versus T at various H. The ambient pressure magnetic measurements indicate the presence of both ferromagnetic and antiferromagnetic exchange interactions with dominant ferromagnetic interactions and with the largest magnetic energy scale ∼ 10 K. The χ(T ) measurements under externally applied pressure of up to P ≈ 1 GPa indicate the robust nature of the spin-liquid state despite relative increase in the ferromagnetic exchanges. C(T ) shows a broad anomaly at T ≈ 2.5 K which moves to higher temperatures in a magnetic field. The evolution of the low temperature C(T, H) and the magnetic entropy is consistent with frustrated magnetism in Ca10Cr7O28. arXiv:1612.02692v2 [cond-mat.str-el]

The elastic behavior of a material can be a powerful tool to decipher thermal transport. In thermoelectrics, measuring the elastic moduli—directly tied to sound velocity—is critical to understand trends in lattice thermal conductivity, as well as study bond anharmonicity and phase transitions, given the sensitivity of elastic moduli to the chemical bonding. In this review, we introduce the basics of elasticity and explain the origin of high‐temperature lattice softening from a bonding perspective. We then review elasticity data throughout classes of thermoelectrics, and explore trends in sound velocity, anharmonicity, and thermal conductivity. We reveal how experimental sound velocities can improve the accuracy of common thermal conductivity models and present a critical discussion of Grüneisen parameter estimates from elastic moduli. Readers will be equipped with tools to leverage elasticity measurements or calculations to accurately interpret thermal transport trends.

Alloys between Mg3Sb2 and Mg3Bi2 have recently been shown to be exceptional thermoelectric materials due in part to their anomalously low thermal conductivity. In the present study, in situ high-pressure synchrotron X-ray diffraction was used to investigate the structure and bonding in Mg3Sb2 and Mg3Bi2 at pressures up to 50 GPa. Our results confirm prior predictions of isotropic in-plane and out-of-plane compressibility but reveal large disparities between the bond strength of the two distinct Mg sites. Using single-crystal diffraction, we show that the octahedral Mg–Sb bonds are significantly more compressible than the tetrahedral Mg–Sb bonds in Mg3Sb2, which lends support to prior arguments that the weaker octahedral Mg bonds are responsible for the anomalous thermal properties of Mg3Sb2 and Mg3Bi2. Further, we report the discovery of a displacive and reversible phase transition in both Mg3Sb2 and Mg3Bi2 above 7.8 and 4.0 GPa, respectively. The transition to the high-pressure structure involves a highly anisotropic volume collapse, in which the out-of-plane axis compresses significantly more than the in-plane axes. Single-crystal diffraction at high pressure was used to solve the monoclinic high-pressure structure (C2/m), which is a distorted variant of the ambient-pressure structure containing four unique Mg coordination environments.

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