Strongly electron correlated system like ABO3 based manganites have revealed exciting physical and structural properties. The origin of above phenomena in these materials is due to competing ordering tendency associated with spin, lattice and charge degrees of freedom, which produces nearly degenerate ground states with markedly different characteristic. They also offer a degree of chemical flexibility, which permits us to control and manipulate its structural, electronic and magnetic states.
Is oxygen vacancy also an important degree of freedom to control and manipulate structure and physical properties of a material?
The quick answer is "yes",
the not so quick answer has me say that I don't consider the notion of "degree of freedom" adequate as far as oxygen stoichiometry is concerned. You may consider the oxygen deficiency a "degree of freedom" when preparing your specimen, and this "degree of freedom" may be within or beyond what you are able to control. But once you have a specific specimen, the oxygen stoichiometry is a given, while the other degrees of freedoms you mention are truly constituents of the physical dynamics in the system.
Oxygen deficiency strongly influences these degrees of freedom, though. It alters the local vibronic properties (disturbs the lattice periodicity), spin properties (it changes the valence of neighboring cations and may hence change their electronic configuration) and it creates a local change in electrostatic potential for carriers.
It could well be, that not few of the exciting properties of oxidic materials discussed today turn out to be a "side effect" of oxygen stoichiometry.
(Going away from the ABO3 materials - some experimenters argue, for example, that the ferromagnetism observed in TM doped ZnO is NOT promarily assotiated with ordering of the TM moments, but with moments of electrons trapped at oxygen vacancy sites)
@Kai Fauth: Thanks for you informative, helpful, knowledgeable comment.
Sir related to the effect of oxygen vacancy on structure and physical properties in perovskite ABO3 based manganites, we have also observed some important results, which I am going to sharing with you.
1: Oxygen-deficiency-induced suppression of JT distortion and stabilization of charge ordering in La0.2Sr0.8MnO3−δ, Aga Shahee, et al., Phys. Status Solidi B, 1–9 (2014),DOI: 10.1002/pssb.201350217
http://onlinelibrary.wiley.com/doi/10.1002/pssb.201350217/abstract
2: Effect of oxygen off-stoichiometry on coupled structural and magnetic phase-transitions in La0.15Sr0.85MnO3−δ (δ=0.02, 0.14); Aga Shahee, et al., Solid State Communications, Volume 177, January 2014, Pages 84–88. http://www.sciencedirect.com/science/article/pii/S0038109813004675
3:Kinetic arrest of the first-order R-3c to Pbnm phase transition in supercooled
LaxMnO3-d (x = 1 and 0.9), Aga Shahee et al.J. Phys.: Condens. Matter 24 (2012) 225405.
http://iopscience.iop.org/0953-8984/24/22/225405/pdf/0953-8984_24_22_225405.pdf
In self-doped La1−xMnO3+δ perovskites, the change of Curie temperature is caused by the change of level of hybridization of Mn 3d states and the O 2p states......Ulyanov et al.,
Article Hybridization of electronic states and magnetic properties o...
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