| Summary: | A novel non-centrosymmetric NdSr<sub>4</sub>O(BO<sub>3</sub>)<sub>3</sub> borate and solid solutions of Nd(Ca<sub>1−<i>x</i></sub>Sr<i><sub>x</sub></i>)<sub>4</sub>O(BO<sub>3</sub>)<sub>3</sub> (<i>x</i> = 0.0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.7, 1.0) were synthesized by solid-state reactions as well as crystallization from a melt. The crystal structures of the Nd(Ca<sub>1−<i>x</i></sub>Sr<i><sub>x</sub></i>)<sub>4</sub>O(BO<sub>3</sub>)<sub>3</sub> solid solutions with <i>x</i> = 0.2, 0.5 and 1.0 were determined from single crystal X-ray diffraction data and refined in the monoclinic space group <i>Cm</i> to <i>R</i><sub>obs</sub> = 0.028, 0.034 and 0.028, respectively. The thermal expansion of the samples with <i>x</i> = 0, 0.2 and 0.5 was investigated using powder high-temperature X-ray diffraction in the temperature range of 25–1000 °C. A similarity of the thermal and compositional (Ca-Sr substitution) deformations of Nd(Ca<sub>1−<i>x</i></sub>Sr<i><sub>x</sub></i>)<sub>4</sub>O(BO<sub>3</sub>)<sub>3</sub> solid solutions is revealed: Heating of Nd(Ca<sub>0.5</sub>Sr<sub>0.5</sub>)<sub>4</sub>O(BO<sub>3</sub>)<sub>3</sub> by 1 °C leads to the same deformations of the crystal structure as increasing the amount of Sr atoms in Nd(Ca<sub>0.5</sub>Sr<sub>0.5</sub>)<sub>4</sub>O(BO<sub>3</sub>)<sub>3</sub> by 0.26 at% Sr. The SHG signal of the series of Nd(Ca<sub>1−<i>x</i></sub>Sr<i><sub>x</sub></i>)<sub>4</sub>O(BO<sub>3</sub>)<sub>3</sub> solid solutions has a maximum at approximately <i>x</i> = 0.2.
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