The evolution of the structural and transport properties of the ${\mathrm{Tl}}_2{\mathrm{Mn}}_2{\mathrm{O}}_7$ pyrochlore and three derivatives obtained by chemical doping at the Tl and Mn sublattices have been studied under high pressure conditions up to 9 GPa. Depending on the relative compressibilities of Tl-O versus Mn-O chemical bonds in ${\mathrm{Tl}}_2{\mathrm{Mn}}_2{\mathrm{O}}_7,{\mathrm{Tl}}_{1.8}{\mathrm{Bi}}_{0.2}{\mathrm{Mn}}_2{\mathrm{O}}_7,{\mathrm{Tl}}_{1.8}{\mathrm{Cd}}_{0.2}{\mathrm{Mn}}_2{\mathrm{O}}_7,$ and ${\mathrm{Tl}}_2{\mathrm{Mn}}_{1.8}{\mathrm{Sb}}_{0.2}{\mathrm{O}}_7$ pyrochlores, a different pressure evolution of the Mn-O bond lengths and Mn-O-Mn angles, governing the magnetic superexchange, has been observed. The transport properties under pressure are also strongly affected by the electronic doping introduced by the chemical substitutions; the formation of frustrated spin systems is observed at high pressures for the Bi- and Cd doped samples, being responsible for a dramatic increase in resistivity. As magnetism and transport are strongly correlated in this system, the variations of the Curie temperature, associated with characteristic metal-to-insulator transitions, are interpreted as a function of the structural changes and the charge carriers density, which varies by orders of magnitude from electron-doped (with Sb) to hole-doped (with Bi, Cd) materials.

• Received 20 September 2002

DOI:https://doi.org/10.1103/PhysRevB.67.104403