RELAXATION DELAY INFLUENCE ON 31P NMR STRUCTURAL CHARACTERIZATION 

R.V.F. Turcu, M. Vasilescu, D. Cacaina, S. Simon

Babes-Bolyai University, Faculty of Physics, Cluj‑Napoca, Romania

Introduction: Nuclear Magnetic Resonance is one of the most important method of material structural characterization.
	The aim of this study is to present the importance of the correct relaxation delay time and method used for MAS NMR measurements, who allow to characterized the system x[Nb2O5·Li2O](100-x)[CaO·P2O5].
	By knowing the local structure in these vitroceramics it is possible to control the LiNbO3 phase development. This kind of vitroceramics samples could be useful for electromagnetic therapy.
Materials and Method. The investigated glass systems x[Nb2O5·Li2O](100-x)[CaO·P2O5] where x=1;5;10;20 mol % are prepared using as starting materials reagent grade chemicals Nb2O5, Li2CO3 , CaCO3, NH4H2PO4. The homogenized mixtures were melted at 12000C for 10 minutes. Glass transition Tg and crystallization Tc temperatures were determined by differential thermal analyses using heating rates of 10oC/min. 
	At this level first 31P MAS NMR measurements are performed using a 400 MHz (9.4T) Bruker spectrometer. The spectra of all four samples are collected at the spin rate of 14 KHz, using Single Pulse, at various pulses length (0.1 – 0.4 s) and corresponding relaxation delay in the range of 0.1 – 0.4 s. All 31P MAS –NMR chemical shifts are expressed in ppm relative to an 85% H3PO4 water solution.  
	The crystalline samples, obtained after heat treatment at 650 C, are now measured at 161.9 MHz with tow different pulse programs. First pulse program it was Single Pulse, with 0.5 – 3 s pulse length, and relaxation delay up to 60 s. The second pulse program used it was Inversion Recovery (t1ir1d). At the spinning rate of 12 kHz, 100 s relaxation delay, 0.5 s delay between 180 and 90 degree pulses, corresponding pulse length for this angles in the rang 1-15 s(depending of attenuation) high resolution 31P MAS NMR spectra are obtained.
Results and Discussion. Using a very long relaxation delay and right pulse program, T1 Inversion Recovery in our case, we have obtain high resolution spectra of 31 P MAS NMR for x[Nb2O5·Li2O](100-x)[CaO·P2O5] vitroceramics  samples.
	The advantage of T1 IR is the fact that all the time you have the response of the same number of spins (all) during our measurement. The evolution of a number of more than 10 different P unites can identified, and also the type of this structure. Four different cubic structures are identified around -7.03 ppm, -8. ppm, -8.5 ppm and –10.1 ppm chemicals shift in all samples. Six different structures are identified in the range of  -23.7, -31.9 ppm chemical shifts.
	The positions and the shape of the signals related to different structures are confirmed by simulation program DMFit.
Conclusion. Utilizing right relaxation delay and NMR pulse program we can very easily to identified and to characterized possible structures in various type of samples, even in the case of many different but closely structures.