School of Physical, Environmental and Mathematical Sciences, The University of New South Wales, ADFA, Canberra, AUSTRALIA^{*} Technische Universitaet Muenchen, FRM II, Garching, Germany^{**}

â—‹Stewart J Campbell^{*} Michael Hofmann^{**}

Rare earth (R) magnetic materials are used extensively in society and have numerous technological applications. When combined with transition metal (T) and metalloid (X) elements, rare earth intermetallic compounds, R-T-X, exhibit a wide range of interesting and useful properties. The prime examples are magnets based on the Nd_{2}Fe_{14}B phase which exhibits the largest energy product (BH) discovered for a magnetic material.

A brief overview of the basic properties of rare earth intermetallic compounds and their applications in modern society will be presented. Interest will then focus on RT_{2}X_{2}, the 1-2-2 series. RMn_{2}X_{2 }compounds are of particular interest as, unlike most transition metals, the Mn atoms carry a magnetic moment. For example, as LaMn_{2}Si_{2} is predominantly ferromagnetic and YMn_{2}Si_{2 }antiferromagnetic; compounds of intermediate composition in La_{1-x}Y_{x}Mn_{2}Si_{2} exhibit mixed magnetic interactions leading to a complex magnetic phase diagram. RMn_{2}(Si,Ge)_{2}** **with R=Yb and Eu exhibit a wide range of unusual properties as well as intermediate valences associated with the transition from a divalent to a trivalent state. An overview of our neutron diffraction investigations of YbMn_{2}(Si,Ge)_{2} (~1.8-723 K; *p *~ 0-2.7 GPa) and EuMn_{2}(Si,Ge)_{2} will also be presented. This has led to a magnetic phase diagram for YbMn_{2}Si_{2-x}Ge_{x} of features consistent with a valence change between the trivalent behaviour of YbMn_{2}Si_{2} and the divalent-like behaviour of YbMn_{2}Ge_{2 }around x_{c }~ 1.6. The volume changes in EuMn_{2}Si_{2} have been analysed for the first time in terms of the interconfigurational fluctuation model, leading to a valence transition temperature of T_{v }~ 527 K, occupation probabilities for the Eu^{2+} state and average Eu^{val} values.