151 Eu ͑and 57 Fe) high-pressure Mössbauer investigations on EuFe 2 P 2 reveal that the pressure-induced isostructural phase transition 3ՇpՇ9 GPa is accompanied by a continuous valence transition from Eu 2ϩ to Eu 3ϩ with the consequence of the disappearance of the Eu͑4 f ) sublattice moment. However, contrary to EuCo 2 P 2 we find in the high-pressure phase of EuFe 2 P 2 no magnetic moment on Fe. Volume-dependent band-structure calculations in both systems show that the formation of the 3d moment strongly depends on the position of the 3d states relative to the Fermi energy and that the filling of the 3d states in EuCo 2 P 2 is mainly caused by the P-3p electrons.The rare-earth (RE) transition metal ͑T͒ phosphides, RET 2 P 2 , crystallizing in the ThCr 2 Si 2 -type structure belong to an interesting class of materials in which structural instability is intimately connected with electronic and magnetic instabilities. These strong electron correlations lead to the formation of a large variety of ground states in this series. Depending on the type of RE and/or T, one observes not only different magnetic phenomena such as 4 f -sublattice magnetism (EuCo 2 P 2 ), 1 3d-sublattice magnetism (LaCo 2 P 2 ) 2 or magnetism of both sublattices (PrCo 2 P 2 ), 3 but also intermediate valent behavior (EuNi 2 P 2 ) 4 or even superconductivity (LaRu 2 P 2 ). 5 Essentially responsible for such a broad variety of properties is the ability of the ThCr 2 Si 2 structure to adapt to several elements with various atomic sizes resulting in extremely different lattice parameters. In fact, within the RET 2 P 2 series, there exists two extremely dissimilar P-P distances along the c axis resulting in two isostructural phases: ␣ phase for a large P-P distance (d P-P Ϸ2.8 Å͒ with a nonbonding P-P state; and ␤ phase for a short d P-P Ϸ2.3 Å forming a P-P single bond state. 6 Structural phase transitions of first-and second-order between these phases can easily be induced by changing composition, 7 temperature, 8 or applying external pressure. 9 According to recent band-structure calculations on some RET 2 P 2 compounds such structural phase transitions strongly affect the electronic structure of these systems. 10 Towards understanding such an interplay in RET 2 P 2 systems, we have recently performed high-pressure experiments ( 151 Eu Mössbauer spectroscpopy and x-ray diffraction͒ on the magnetically ordered compound EuCo 2 P 2 . 11 In this system, antiferromagnetism is due to ordering of the Eu 2ϩ ͑ 8 S 7/2 ) sublattice moments (T N ϭ66.5 K͒ while the Co(3d) sublattice exhibits no magnetic moments. Here, high-pressure experiments demonstrated that the pressure-induced first-order structural ͑␣→␤͒ phase transition at pϾ3 GPa is connected with a sharp valence transition of Eu from magnetic Eu 2ϩ to a nonmagnetic Eu 3ϩ ( 7 F 0 ) state. As a consequence, the disappearance of the Eu(4 f ) sublattice magnetism is accompanied by a simultaneous appearance of Co(3d) sublattice magnetism with T N *ϭ260 K. 11 These experimental findings, however, rais...