The detection of ionizing radiation is important in numerous applications related to national security ranging from the detection and identification of fissile materials to the imaging of cargo containers. A key performance criterion is the ability to reliably identify the specific gamma-ray signatures of radioactive elements, and energy resolution approaching 2% at 662 keV is required for this task. In this work, we present discovery and development of new high energy resolution scintillators for gamma-ray detection. The new ternary halide scintillators belong to the following compositional families: AM2X5:Eu, AMX3, and A2MX4:Eu (A = Cs, K; M = Ca, Sr, Ba; X = Br, I) as well as mixed elpasolites Cs2NaREBr3I3:Ce (RE = La, Y). Using thermal analysis, we confirmed their congruent melting and determined crystallization and melting points. Using the Bridgman technique, we grew 6, 12 and 22 mm diameter single crystals and optimized the Eu concentration to obtain the best scintillation performance. Pulse-height spectra under gamma-ray excitation were recorded in order to measure scintillation light output, energy resolution and light output nonproportionality. The KSr2I5:Eu 4% showed the best combination of excellent crystal quality obtained at fast pulling rates and high light output of ~95,000 photons/MeV with energy resolution of 2.4% at 662 keV.