Crystal Chemistry and Optical Spectroscopy of the Monophosphate Series A^ICd_1-xM^II_xPO₄(A^I=Li, Na; M^II=Mn, Co)

抄録

Continuous solid solutions were isolated within the system A^ICd_1-xM^I_xPO₄(A^I=Li, Na; M^II=Mn, Co) for 0≤x≤1. The steady decrease of the lattice parameters and unit cell volume within the solid solution domains was ascribed to the substitution of Cd²⁺ cations by smaller size M²⁺ ions (M=Mn, Co) as the composition progresses between x=0 and x=1. The investigation of the luminescence of Mn(II) has evidenced two large emission bands peaking at circa 576-580nm and 693-710nm for the analyzed compounds LiCd_1-xMn_xPO₄(0≤x≤0.7) under the excitation radiation with λ_exc=438nm. In this system, a small red shift seems to appear for both emission bands when a higher excitation energy is used (λ_exc=325nm). In the case of LiZn_1-xMn_xPO₄(0≤x≤0.2), only one luminescence band was recorded within a narrower range 550-556nm for λ_exc=488nm and at ca. 562nm under λ_exc=325nm. For the composition analysed in the system NaCd_1-xMn_xPO₄(x=0.05), the emission bands were found at 624.5nm and 616.4nm under excitation wavelengths equal to 488 and 325nm respectively. The luminescence of Mn²⁺ ions in all investigated phases was attributed to the same electronic transition ⁴T_1g(⁴G) ⁶A_1g(⁶S) which is very sensitive to the crystal filed strength as evidenced by Tanabe-Sugano energy diagram. Indeed as the solid solution progresses in the case of LiCd_1-xMn_xPO₄ for example (0≤x≤1), the size of the cavities hosting Mn²⁺ ions is consequently changing. As a matter of fact, the mean manganese-oxygen distance is expected to vary between the values corresponding to the mean bond length of the divalent cation in LiCdPO₄(<Cd-0>=2.298Å) and LiMnPO₄(<Mn-O>=2.201Å) respectively. The resulting change of the crystal field around Mn(II) may explain why the emission is observed in the green region for low values of x while it shifts to the red domain for compositions close to LiMnPO₄(x>0.5). The correlation between the modification of the wavelength of Mn²⁺ luminescence bands and the crystal field strength inherent to the variation of the chemical composition is therefore established. A model for the distribution of the divalent cations within the lattice along the solid solutions was also proposed.