Intermetallic Charge Transfer in V-Substituted PbCrO₃

Abstract

PbCrO 3 features an unusual charge distribution Pb 0.5 2+ Pb 0.5 4+ Cr 3+ O 3 with Pb charge disproportionation at ambient pressure. A charge transfer between Pb and Cr is induced by the application of pressure resulting in Pb 2+ Cr 4+ O 3 charge distribution and a large volume collapse. Here, structural and charge distribution changes in PbCr 1−x V x O 3 are investigated. Despite a cubic crystal structure in 0 ≤ x ≤ 0.60, discontinuous reduction in the unit cell volume was observed between x = 0.35 and 0.40. Hard X-ray photoemission spectroscopy confirmed the change in Pb charge state from the coexisting Pb 2+ and Pb 4+ at x = 0.35 to single Pb 2+ at x = 0.40. This indicates that V substitution stabilizes the high pressure cubic Pb 2+ Cr 4+ O 3 -type phase. With further increase in the V substitution, the PbVO 3 -type polar tetragonal phase appeared at x = 0.80.

Two Types of Negative Thermal Expansion Observed in PbCr1–xTixO3

“…We succeeded in significantly lowering the working temperature of PbTiO 3 as an NTE material by Cr substitution while retaining a large volume shrinkage of 0.6%, though the temperature-induced charge transfer we originally expected did not occur. In contrast to the study of V 4+ -substituted system, PbCr 1– x Ti x O 3 shows NTEs in all compositions, and two NTE regions are adjacent (Figure d). Our findings in this study should pave the way for designing new NTE materials and will further stimulate the ongoing quest for identifying such materials.…”

“…PbCr 1– x V x O 3 shows the cubic-to-cubic isostructural phase transition between x = 0.35 and 0.4 accompanied by a large discontinuous volume collapse of 3.5%, which strongly suggests composition-induced intermetallic charge transfer from Pb 2+ (1+ x )/2 Pb 4+ (1– x )/2 Cr 3+ 1– x V 4+ x O 3 to Pb 2+ Cr 4+ 1– x V 4+ x O 3 . There is another possibility that the valence of V changes instead of Cr, but the disappearance of Pb 4+ has been confirmed by HAXPES . In contrast, there is no such distinct volume change within the cubic phase of PbCr 1– x Ti x O 3 , implying the absence of the valence change from Cr 3+ to Cr 4+ .…”

“…There is another possibility that the valence of V changes instead of Cr, but the disappearance of Pb 4+ has been confirmed by HAXPES. 40 In contrast, there is no such distinct volume change within the cubic phase of PbCr 1−x Ti x O 3 , implying the absence of the valence change from Cr 3+ to Cr 4+ . A slight monotonic decrease of lattice volumes as x increases comes from the smaller ionic radius of Ti 4+ (0.605 Å) compared to that of Cr 3+ (0.615 Å).…”

“…Their sample was synthesized at 4 GPa, which is much lower than our synthetic pressure, so we attribute the phase coexistence observed in their study to a compositional phase separation between PbCr 0.7+δ Ti 0.3−δ O 3 and PbCr 0.7−δ Ti 0.3+δ O 3 due to the solubility limit at 4 GPa. Figure shows the composition dependence of the unit cell volume and the lattice parameters of PbCr 1– x Ti x O 3 at RT along with those of PbCr 1– x V x O 3 quoted from ref for a comparison of the same tetravalent ion substitutions for Cr. PbCr 1– x V x O 3 shows the cubic-to-cubic isostructural phase transition between x = 0.35 and 0.4 accompanied by a large discontinuous volume collapse of 3.5%, which strongly suggests composition-induced intermetallic charge transfer from Pb 2+ (1+ x )/2 Pb 4+ (1– x )/2 Cr 3+ 1– x V 4+ x O 3 to Pb 2+ Cr 4+ 1– x V 4+ x O 3 .…”

“…PbCr 1– x Ti x O 3 does not show the discontinuous volume shrinkage observed in the cubic phases of PbCr 1– x V x O 3 , indicating the absence of valence change from Cr 3+ to Cr 4+ . The data of PbCr 1– x V x O 3 are quoted with permission from ref . Copyright 2021 American Chemical Society.…”