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Abstract The present work concerns calcium pyroselenites CaSe 2 O 5 with two independent crystal structures, their difference physical properties and phase transformations. α-CaSe 2 O 5 in orthorhombic form shows 2D calcium oxide anionic layers whereas new compound β-CaSe 2 O 5 crystallizes in monoclinic space group C 2 /c. It's a new structure type with 1D calcium oxide thick chains which are further connected by Se 2 O 5 dimers. Rietveld refinement indicates that the weight fraction for α/β-CaSe 2 O 5 mixture phase was found to be 84.5% and 15.5% for α- and β-CaSe 2 O 5 , respectively. Optical diffuse reflectance spectrum coupled with Kubelka–Munk fit gives a result that α-CaSe 2 O 5 adopts direct optical transition model while β-CaSe 2 O 5 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-CaSe 2 O 5 , respectively. Thermal analysis coupled with variable temperature powder XRD indicates the phase transformation between these two phases. Graphical abstract β-CaSe 2 O 5 features a new structure which was reported for the first time. Compare with hyper-symmetric of α-CaSe 2 O 5 , structural differences lead into the changes for their physical performances. Optical properties analysis indicates that α-CaSe 2 O 5 is a direct bandgap semiconductor whereas β-CaSe 2 O 5 features an indirect optical transition. Thermal analysis coupled with variable temperature powder XRD indicates the phase transformation of two phases. fx1 Highlights • β-CaSe 2 O 5 was reported for the first time. • Pure α-CaSe 2 O 5 was obtained under a rapid microwave-assist reaction conditions. • α-CaSe 2 O 5 features a direct optical transition model whereas β-CaSe 2 O 5 is an indirect semiconductor. • Thermal analysis coupled with variable temperature powder XRD indicates the phase transformation of two phases. [ABSTRACT FROM AUTHOR]