Crystal Structure and Photoluminescence Properties of an Incommensurate Phase in EuO- and P 2 O 5 -Doped Ca 2 SiO 4 .

We have for the first time clarified the incommensurately modulated crystal structure as well as the photoluminescence properties of Eu ²⁺ -activated Ca ₂ SiO ₄ solid solution, the chemical formula of which is (Ca _1.88 Eu ²⁺ _0.01 □ _0.11 )(Si _0.78 P _0.22 )O ₄ , where □ denotes vacancies in Ca sites with the replacement of Si ⁴⁺ by P ⁵⁺ . The emission spectrum upon the 335 nm excitation showed a relatively broad band centered at ca. 490 nm and a full width at half-maximum of ca. 80 nm. The crystal structure was made up of the four types of β-Ca ₂ SiO ₄ -like layers with one type of interlayer. The incommensurate modulation with superspace group Pnma(0 β 0)00 s was induced by the long-range stacking order of these layers. The modulation wavevector was 0.27404(2) × b*, with the basic unit-cell dimensions being a = 0.68355(2) nm, b = 0.54227(2) nm, and c = 0.93840(3) nm ( Z = 4). The basic structure contained two nonequivalent Ca sites. One site was fully occupied by Ca ²⁺ and free from Eu ²⁺ in the overall incommensurate structure. The occupational modulation at the other site was so significant that the sum of site occupation factors for Ca ²⁺ and Eu ²⁺ as low as 0.5 was seen at the interlayer. This site was too large for accommodation of Ca ²⁺ but was suitable for Eu ²⁺ . Thus, the Eu ²⁺ ions would exclusively concentrate at the relevant site, which would cause the emission peak of the incommensurate phase to be shifted to the shorter wavelength ranges as compared with those of the other commensurate phases such as β and α' _L .