Elastic behavior near phase transitions with negative dP/dT

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Abstract
Sound velocities have been measured in polycrystalline samples of silver iodide at 122°C to pressures of 6 × 108 N/m2. The data cover three different polymorphs within their respective stability fields. The low‐pressure phase (B4 structure) has pronounced negative pressure derivatives of both velocities, probably due to defects. The intermediate pressure phase (B23 structure) stable from ∼2 to ∼3.5 × 108 N/m2, has anomalously low velocities and elastic moduli for its density. The high‐pressure phase (B1 structure) has anomalously high dVp/dP and dK/dP, also possibly due to defects. The velocity and bulk modulus shifts at these transitions, especially the B4‐B23 transition, are in accordance with a previous prediction of anomalous velocity shifts to be expected at a transition with a negative dP/dT.

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