We perform an extensive study of Sr_{3}Cr_{2}O_{7}, the n=2 member of the Ruddlesden-Popper Sr_{n+1}Cr_{n}O_{3n+1} system. An antiferromagnetic ordering is clearly visible in the magnetization and the specific heat, which yields a huge transition entropy, Rln(6). By neutron diffraction as a function of temperature we have determined the antiferromagnetic structure that coincides with the one obtained from density functional theory calculations. It is accompanied by anomalous asymmetric distortions of the CrO_{6} octahedra. Strong coupling and Lanczos calculations on a derived Kugel-Khomskii Hamiltonian yield a simultaneous orbital and moment ordering. Our results favor an exotic ordered phase of orbital singlets not originated by frustration.

We have performed neutron diffraction measurements of tetragonal Sr2CrO4 to study the crystallographic structure as a function of temperature and the magnetic properties developed below . We have measured the precise positions of the oxygen atoms of the CrO6 octahedra and observed that, with decreasing temperature, the octahedra pass from elongated to almost ideal, contradicting the intuitive idea of a Jahn-Teller effect. DFT calculations explain this apparent anomalous behaviour by establishing that energy level degeneracy only exists in a highly elongated octahedra. Our study agrees with the idea of a weak orbital ordering transition around 150 K followed by an itinerant antiferromagnetic ordering at 110 K with a wave vector determined both from neutron powder diffraction and theory.

We report on the structure and physical properties of a bidimensional chromate, Sr 4 Cr 3 O 10 , the n=3 member of the Ruddlesden-Popper Sr n+1 Cr n O 3n+1 series. For the first time, using complementary x-ray powder diffraction and electron diffraction data, we have solved its layered crystallographic structure. Our study shows also that this high pressure phase is insulating and antiferromagnetic below T N =280K, a similar behavior already observed for n=1, 2 and n=+∞ members.