X-ray observation of crossover of 2kF to 4kF scattering in (N-Methylphenazinium)x(Phenazine)1-x(Tetracyanoquinodimethane) [(NMP)x(Phen)1-x(TCNQ)], 0.5 (< _x <_ 1.0)

We report the temperature-dependent x-ray diffuse scattering of (NMP),(Phen)(1-x-)(TCNQ) as a function of conduction-electron density (x). With decreasing x three different unique electronic instability regimes are successively observed on the TCNQ chains: (i) the 2kF instability for (2/3<_x<_1, (ii)both 2kF and 4 kF instabilities for 0.57(<_x<_2/3, and (iii) the 4kF instability alone for x close to 0.5. These results provide direct evidence for the critical role of interchain screening in determining the magnitude of the effective Coulomb interaction.