Phase-alternating decoupling (XiX) [1,2] is one of the simplest and efficient high-power decoupling methods in solid-state NMR for faster MAS frequencies. It consists of two pulses of equal length but with a phase difference of 180°. We present variations of this heteronuclear decoupling sequences with improved performance based on numerical calculations of the second-order cross terms . By changing either the amplitude or the length of one of the two pulses, the effective nutation angle of the basic two-pulse element of XiX is changed from 0 to 180°. Such a modification leads to narrower lines compared to the standard high-power XiX scheme. We show experimentally that the two options to implement the effective 180° flip angle, either by amplitude or by pulse-length variation, lead to similar results and significantly longer T2' times compared to standard XiX. For both sequences the optimum parameters can be calculated from experimental settings with simple analytical expressions, resulting in a decoupling scheme, which is easy to set up.
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