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We examine the effect of small amounts of magnetic substituents in the $A$sites of the frustrated spinels MgCr$_2$O$_4$ and ZnCr$_2$O$_4$. Specificallywe look for the effects of spin and lattice disorder on structural changesaccompanying magnetic ordering in these compounds. Substitution of Co$^{2+}$ onthe non-magnetic Zn$^{2+}$ site in Zn$_{1-x}$Co$_{x}$Cr$_2$O$_4$ where0\,$<$\,$x$\,$\leq$\,0.2 completely suppresses the spin-Jahn-Teller distortionof ZnCr$_2$O$_4$ although these systems remain frustrated, and magneticordering occurs at very low temperatures of $T$\,$<$\,20\,K. On the other hand,the substitution of Jahn-Teller active Cu$^{2+}$ for Mg$^{2+}$ and Zn$^{2+}$ inMg$_{1-x}$Cu$_{x}$Cr$_2$O$_4$ and Zn$_{1-x}$Cu$_{x}$Cr$_2$O$_4$ where0\,$<$\,$x$\,$\leq$\,0.2 induce Jahn-Teller ordering at temperatures well abovethe N\'eel temperatures of these solid solutions, and yet spin interactionsremain frustrated with long-range magnetic ordering occurring below 20\,Kwithout any further lattice distortion. The Jahn-Teller distorted solidsolutions Mg$_{1-x}$Cu$_{x}$Cr$_2$O$_4$ and Zn$_{1-x}$Cu$_{x}$Cr$_2$O$_4$ adoptthe orthorhombic $Fddd$ structure of ferrimagnetic CuCr$_2$O$_4$. Total neutronscattering studies of Zn$_{1-x}$Cu$_{x}$Cr$_2$O$_4$ suggest that there arelocal $A$O$_4$ distortions in these Cu$^{2+}$-containing solid solutions atroom temperature and that these distortions become cooperative when averagestructure distortions occur. Magnetism evolves from compensatedantiferromagnetism in MgCr$_2$O$_4$ and ZnCr$_2$O$_4$ to uncompensatedantiferromagnetism with substitution of magnetic cations on the non-magneticcation sites of these frustrated compounds.

Structural ground states of ($A,A'$)Cr$_2$O$_4$($A$=Mg, Zn; $A^{\prime}$ = Co, Cu) spinel solid solutions: Spin-Jahn-Teller and Jahn-Teller effects