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Abstract The magnetic phase diagram of Sr $$_{1-x}$$ 1 - x Ca $$_x$$ x Co $$_2$$ 2 P $$_2$$ 2 as a function of hydrostatic pressure and temperature is investigated by means of high pressure muon spin rotation, relaxation and resonance ( $$\mu ^+$$ μ + SR). The weak pressure dependence for the $$x\ne 1$$ x ≠ 1 compounds suggests that the rich phase diagram of Sr $$_{1-x}$$ 1 - x Ca $$_x$$ x Co $$_2$$ 2 P $$_2$$ 2 as a function of x at ambient pressure may not solely be attributed to chemical pressure effects. The $$x=1$$ x = 1 compound on the other hand reveals a high pressure dependence, where the long range magnetic order is fully suppressed at $$p_{\rm{c2}}\approx 9.8$$ p c 2 ≈ 9.8  kbar, which seem to be a first order transition. In addition, an intermediate phase consisting of magnetic domains is formed above $$p_{\rm{c1}}\approx 8$$ p c 1 ≈ 8  kbar where they co-exist with a magnetically disordered state. These domains are likely to be ferromagnetic islands (FMI) and consist of an high- (FMI- $$\textcircled {1}$$ 1 ) and low-temperature (FMI- $$\textcircled {2}$$ 2 ) region, respectively, separated by a phase boundary at $$T_{\rm{i}}\approx 20$$ T i ≈ 20  K. This kind of co-existence is unusual and is originating from a coupling between lattice and magnetic degrees of freedoms.