Orbital- and spin-driven lattice instabilities in quasi-one-dimensional CaV2 O4

Calcium vanadate CaV2O4 has a crystal structure of quasi-one-dimensional zigzag chains composed of orbital-active V3+ ions and undergoes successive structural and antiferromagnetic phase transitions at Ts∼140 K and TN∼70 K, respectively. We perform ultrasound velocity measurements on a single crystal of CaV2O4. The temperature dependence of its shear elastic moduli exhibits huge Curie-type softening upon cooling that emerges above and below Ts depending on the elastic mode. The softening above Ts suggests the presence of either onsite Jahn-Teller-type or intersite ferrotype orbital fluctuations in the two inequivalent V3+ zigzag chains. The softening below Ts suggests the occurrence of a dimensional spin-state crossover, from quasi-one to three, that is driven by the spin-lattice coupling along the inter-zigzag-chain orthogonal direction. The successive emergence of the orbital- and spin-driven lattice instabilities above and below Ts, respectively, is unique to the orbital-spin zigzag chain system of CaV2O4.