Neodymium doped yttrium aluminum garnet (Nd:YAG or (Y_1−xNd_x)₃Al₅O₁₂) is one of the most important laser materials. Crystals are available from different commercial suppliers with dopant concentrations up to 2.5 at% (x ≤ 0.025). The crystals are grown from the melt usually by the Czochralski method. Besides, in the framework of this study Nd:YAG crystals with different doping levels are grown by the micro-pulling-down method. For both methods, the material has to be kept at the melting point that is for undoped (x=0) Y₃Al₅O₁₂ T_f = 1940°C and drops only slightly for x>0. Alternatively, single phase (Y_1−xNd_x)₃Al₅O₁₂ powders with doping levels as high as x = 0.275 could be produced in a low-temperature sol-gel process. DTA measurements up to T_f showed that the highly doped Nd:YAG powders are in thermodynamic equilibrium only up to (depending on x) T ≅ 1650°C and decompose irreversibly at higher T. These results can be understood if the crystals are considered as solid solutions within the concentration triangle Al₂O₃ – Y₂O₃ – Nd₂O₃. Based such treatment a pseudo-binary "garnet" section of this concentration triangle will be proposed that explains the experimental findings known so far.