Although SrNi2βP2β adopts the common ThCr2βSi2β structure for Tβ₯325 K, being in an uncollapsed tetragonal (ucT) state, on cooling below 325 K
it adopts a one-third collapsed orthorhombic (tcO) phase where one out of every
three P-rows bond across the Sr layers. On the other hand, SrCo2βP2β only
exhibits the uncollapsed ThCr2βSi2β structure from room temperature down to
1.8 K. Neither SrNi2βP2β nor SrCo2βP2β manifest magnetic transitions
down to 50 mK and 2 K, respectively. In this work we report the effects of Co
substitution in Sr(Ni1βxβCoxβ)2βP2β, which allows for tuning the
transition between the one-third collapsed and the uncollapsed structure. We
find a rapid decrease of the one-third collapsed structural transition
temperature with increasing Co fraction, until reaching full suppression for xβ₯0.1. Substitution levels in the range 0.11β€xβ€0.58 show no signs
of any transition down to 1.8 K in the magnetization or resistance measurements
in the range 1.8Β Kβ€Tβ€300Β K. However, different
magnetically ordered states emerge for xβ₯0.65, and disappear for xβ₯0.99, recovering the known paramagnetic properties of the parent compound
SrCo2βP2β. These results are summarized in a phase diagram, built upon the
characterization done on single crystals with different Co fraction. Both the
magnetic and structural properties are compared to other systems with
ThCr2βSi2β structure that exhibit magnetic ordering and collapsed
tetragonal transitions. The magnetic ordering and moment formation are well
described by Takahashi's spin fluctuation theory of itinerant electron
magnetism.Comment: 17 pages, 20 figure