Order and Disorder: Toward the Thermodynamically Stable α-BaMoO₂F₄ from the Metastable Polymorph

Abstract

A fully ordered noncentrosymmetric barium molybdenum oxyfluoride, α-BaMoO2F4, has been synthesized by a hydrothermal reaction at 200 °C in a concentrated hydrofluoric acid solution. A centrosymmetric polymorph with O/F disorder, β-BaMoO2F4, has been obtained in several minutes when the reaction mixture was stirred at room temperature in the same medium. Interestingly, we found that the metastable β-BaMoO2F4 transforms into the thermodynamically stable α-BaMoO2F4 in an ambient condition. More detailed kinetic studies using powder X-ray diffraction indicate that the MoO2F4 octahedra in the kinetic phase, β-BaMoO2F4, rearrange through the constant dissolution/precipitation process to find a more stable orientation and form the fully ordered α-BaMoO2F4. Density functional theory (DFT) calculations suggest that the formation of the thermodynamically stable α-BaMoO2F4 is driven by the strong Mo–O π-interactions induced by Ba2+ cations. The new finding on the understanding of the kinetics of solid-state reactions suggests a novel way toward an effective discovery of functional materials with asymmetric structures.

Metal oxyhalides: an emerging family of nonlinear optical materials

“…In addition, quite a few “pure” d 0 /d 10 transition metal oxyfluorides with NLO properties, such as Sr 3 Nb 2 O 2 F 12 ·2H 2 O, 57 SrMoO 2 F 4 , 58 α-BaMoO 2 F 4 , 58 PbMoO 2 F 4 , 58 KMoO 2 F 3 , 59 NaVO 2− x F 2+ x , 60 and KNaNbOF 5 , 61 have also been discovered recently. However, most of them exhibit relatively small SHG responses (≤1× KDP) possibly attributed to the relatively small distortions or the unfavorable arrangements of their transition metal–oxyfluoride functional units.…”

An Effective Strategy for Designing Nonlinear Optical Crystals by Combining the Structure‐Directing Property of Oxyfluorides with Chemical Substitution

“…With these structure‐directing properties, many NCS and polar structures have been rationally designed. However, it should be noted that the NCS structure cannot guarantee the material to possess a large SHG response, for example, the NCS KNaNbOF 5 (0.6×KDP) [35] and α‐BaMoO 2 F 4 (1×SiO 2 , one order smaller than that of KDP), [36] both exhibit relatively small SHG responses. So, it is important for designing new NLO crystals to further enhance their SHG responses by the structural modification.…”

Uncovering the Structural Diversity and Excellent Performance of a Deep Ultraviolet Nonlinear Optical System Li(B2O3)nF (n= 1, 1.5, 2, and 3) by Multicomponent Prediction

“…Nonlinear optical (NLO) crystals have very important applications in many advanced domains, especially in solid-state lasers, and drawn considerable attention owing to their frequency-doubling properties. − Deep ultraviolet (deep-UV) lasers with wavelengths below 200 nm have high photonic energy, high photonic flux, and high spectral resolution and are an important part of laser-driven interferometric lithography, precision micromachining, detection, and spectral analysis. − KBe 2 BO 3 F 2 (KBBF) and its isomorphic RbBe 2 BO 3 F 2 are the only crystals that can directly achieve deep-UV output of 1064 nm Nd:YAG laser by second harmonic generation (SHG). , For deep-UV NLO materials, a large band gap ( E g ≥ 6.2 eV), relatively strong SHG response ( d ij ≥ that of the UV benchmark KH 2 PO 4 (KDP), 0.39 pm/V), and suitable birefringence (Δ n ) for phase matching have been pursued for a long time. To enrich the structure and tune the optical performance, the introduction of halogen into borates becomes one strategy .…”