Changes of optical transition models caused by crystal structural changes in CaSe2O5

• Published in: Journal of solid state chemistry 2019-03, Vol.271, p.115-120
• Main Authors: Zhang, Jian-Han, Lin, Ming-Shui, Zhao, Meng-Yang, Zhang, Meng, Chang, Jin-Yin, Ying, Shao-Ming
• Format: Article
• Language: English
• Subjects: Optical bandgap; Phase transformation; Rietveld refinement; Selenite
• ISSN: 0022-4596; 1095-726X
• DOI: 10.1016/j.jssc.2018.12.056

The present work concerns calcium pyroselenites CaSe2O5 with two independent crystal structures, their difference physical properties and phase transformations. α-CaSe2O5 in orthorhombic form shows 2D calcium oxide anionic layers whereas new compound β-CaSe2O5 crystallizes in monoclinic space group C2/c. It's a new structure type with 1D calcium oxide thick chains which are further connected by Se2O5 dimers. Rietveld refinement indicates that the weight fraction for α/β-CaSe2O5 mixture phase was found to be 84.5% and 15.5% for α- and β-CaSe2O5, respectively. Optical diffuse reflectance spectrum coupled with Kubelka–Munk fit gives a result that α-CaSe2O5 adopts direct optical transition model while β-CaSe2O5 is an indirect semiconductor. The experimental result matches well with the prediction of theory calculation. The optical bandgap values were fitted to be 4.74 and 4.12 eV for α- and β-CaSe2O5, respectively. Thermal analysis coupled with variable temperature powder XRD indicates the phase transformation between these two phases. β-CaSe2O5 features a new structure which was reported for the first time. Compare with hyper-symmetric of α-CaSe2O5, structural differences lead into the changes for their physical performances. Optical properties analysis indicates that α-CaSe2O5 is a direct bandgap semiconductor whereas β-CaSe2O5 features an indirect optical transition. Thermal analysis coupled with variable temperature powder XRD indicates the phase transformation of two phases. [Display omitted] •β-CaSe2O5 was reported for the first time.•Pure α-CaSe2O5 was obtained under a rapid microwave-assist reaction conditions.•α-CaSe2O5 features a direct optical transition model whereas β-CaSe2O5 is an indirect semiconductor.•Thermal analysis coupled with variable temperature powder XRD indicates the phase transformation of two phases.