Variation in the nature of the neutral-ionic transition in DMTTF − QCl 4 under pressure probed by NQR and NMR

• Published in: Physical review. B 2019-03, Vol.99 (12), Article 125133
• Main Authors: Sunami, K., Iwase, F., Miyagawa, K., Horiuchi, S., Kobayashi, K., Kumai, R., Kanoda, K.
• Format: Article
• Language: English
• Subjects: Antiferroelectricity; Charge transfer; Dimerization; NMR; Nuclear magnetic resonance; Phase diagrams; Phase transitions; Pressure; Variation
• ISSN: 2469-9950; 2469-9969
• DOI: 10.1103/PhysRevB.99.125133

We have investigated the microscopic charge/lattice/spin states in the neutral-ionic transition material DMTTF − QCl 4 under pressure by 35 Cl − NQR and 1 H − NMR measurements to reveal the mechanism of the phase transition and the nature of the pressure-temperature phase diagram. At ambient pressure, the 35 Cl − NQR frequency indicates low ionicity and shows neither discontinuous jump nor precursory decrease around the transition, suggesting that the spin degrees of freedom are not involved in a dimerization transition. These experimental features are consistent with the theoretically proposed picture that the electronic energy gain is acquired by modulations in transfer integrals (so-called Peierls mechanism) with the lattice dimerization. As pressure is applied, the ionicity is increased and the system is driven into an ionic state with the degree of charge transfer exceeding 0.5 above ∼ 9 kbar at room temperature, where the 1 H − NMR captures paramagnetic spin fluctuations indicative of the emergence of a nondimeric ionic phase. Furthermore, 35 Cl − NQR and x-ray-diffraction measurements found a distinct antiferroelectric state at high pressures above 11 kbar. Based on these results, we propose a pressure-temperature phase diagram.