Effect of pressure on the phase transition of the [(C6H5)3PCH3]+(TCNQ) ˙–2 anion radical salt as studied by differential thermal analysis under pressure

Abstract

The anion radical salt of [(C₆H₅)₃PCH₃]⁺(TCNQ)^˙–₂, where TCNQ is 7,7,8,8-tetracyanoquinodimethane, undergoes a solid–solid phase transition at 315.7 K at atmospheric pressure, where sharp discontinuities were found in the temperature dependences of the electrical conductivity and magnetic susceptibility. In the present paper, the effect of pressure on this phase transition has been studied by differential thermal analysis (d.t.a.) in the pressure range up to 200 MPa. The transition temperature (T_p/K) is related to the pressure (p/MPa) by the expression T_p= 315.5₈–7.2₆× 10^–2p–8.1₀× 10^–4p²–1.1₈× 10^–6p³. The mechanism of the phase transition is discussed on the basis of these results together with the crystal structure data.