Chemical pressure tuning of competing orders in ${\text{Ba}}_{1-x}{\text{Ca}}_x{\text{Ni}}_2{\text{As}}_2$

$\mathrm{Ba}{\mathrm{Ni}}_2{\mathrm{As}}_2$, a structural-analogue to the iron-based parent compound $\mathrm{Ba}{\mathrm{Fe}}_2{\mathrm{As}}_2$, offers a unique platform to study the interplay between superconductivity, charge density waves and, possibly, electronic nematicity. Here, we report on the growth and characterization of ${\mathrm{Ba}}_{1-x}{\mathrm{Ca}}_x{\mathrm{Ni}}_2{\mathrm{As}}_2$ single crystals with $0\le x\le 0.1$, using a combination of x-ray diffraction, diffuse x-ray scattering, heat capacity, and electronic transport measurements. Our results demonstrate that calcium substitution affects the structural, electronic and thermodynamic properties of $\mathrm{Ba}{\mathrm{Ni}}_2{\mathrm{As}}_2$ in a way that is strongly reminiscent of moderate hydrostatic pressures albeit with marked differences. In particular Ca-substitution efficiently suppresses both the triclinic structural transition and the associated commensurate charge density wave formation, while increasing the superconducting transition temperature. We found that the substitution range in which the crystals remain homogeneous is limited as for concentrations $x\ge 0.04$ intense diffuse x-ray scattering indicates the formation of stacking faults, which, despite the preserved integrity of the NiAs layers, prevents investigation up to concentrations at which the chemical pressure would completely suppress the structural instability.