Stabilizing PrBO ₄ (B = Cr, V) Monazite Polymorphs by Phosphate Loading: Chemical Pressure Perspective with Insights into Optical and Thermochemical Oxygen-Storage Properties of 3d ¹ (Cr ⁵⁺ )

Rational control of polymorph selection, particularly for metastable phases typically accessible only under extreme conditions, remains a pressing challenge. Here, we demonstrate a simple polyanion-substitution strategy to stabilize PrCrO 4 (PrCr 1– x P x O 4 ( x = 0.075–1.00) and high-pressure PrVO 4 (PrP 1– x V x O 4 ( x = 0.00–0.40)) in the monazite (S.G. P 2 1 / n ) structure under ambient conditions. In addition to powder X-ray diffraction, the structure was confirmed by using high-resolution transmission electron microscopy and Raman spectral measurements. A medium-entropy system, PrCr 1/3 P 1/3 V 1/3 O 4, was fabricated to investigate the phase stability drivers. Optical investigations of PrCr 1– x P x O 4 ( x = 0.075–1.00) highlighted the role of distorted C 1 tetrahedral symmetry in Cr 5+ d–d transitions, yielding a color comparable to that of the standard inorganic green pigment Cr 2 O 3 at a lower chromium content. The metastability of Cr 5+ enables a low-temperature reversible oxygen-storage capability in PrCr 0.50 P 0.50 O 4 . The Cr 5+ /Cr 4+ redox interplay underpinned the oxygen sorption and desorption. The chemical pressure in these systems was confirmed and quantified approximately using the available hydrostatic pressure coefficients and Raman mode hardening, with certain assumptions made. Furthermore, the successful transformation of zircon-CeVO 4 to a monazite structure when 50 mol % of VO 4 3– was substituted with PO 4 3– demonstrates the applicability of this strategy for realizing metastable polymorphs in [Pr/Ce](V, Cr, P)O 4 systems under ambient conditions.